ENH Enable streaming from S3
File reading from AWS S3: Modify the `get_filepath_or_buffer` function such that it only opens the connection to S3, rather than reading the entire file at once. This allows partial reads (e.g. through the `nrows` argument) or chunked reading (e.g. through the `chunksize` argument) without needing to download the entire file first.

I wasn't sure what the best place was to put the `OnceThroughKey`. (Suggestions for better names welcome.) I don't like putting an entire class inside a function like that, but this keeps the `boto` dependency contained.

The `readline` function, and modifying `next` such that it returns lines, was necessary to allow the Python engine to read uncompressed CSVs.

The Python 2 standard library's `gzip` module needs a `seek` and `tell` function on its inputs, so I reverted to the old behavior there.

Partially addresses #11070 .

New commit addresses your comments. I'll squash once the review is complete.
 need to change class name references

add a test using chunk size 
 Will do. BTW -- reading with `nrows` or `chunksize` worked before, too. The difference now is that we can do it without having to download the entire file. I don't know how to write a unit test for that -- I tested it myself by looking at whether it's 100 ms or 20 s to read part of a large file. In this case, I think it's okay just to check for functionality. The way the code is written, the only way we'd lose partial reads without breaking tests is for downstream code to suck in the entire file before feeding it to the parser. (This happens right now for bz2 files without the changes in #11072 .)
 in that case why don't u make an asv benchmark and read say 100 lines or something
thrn we would have a perf regression if this was changed (don't use a really long file but should take maybe 1s) to download the entire thing
 I think it would need about 1 MM rows, maybe more, to be able to reliably separate performance regression from variance in network performance. I'd also need a public S3 bucket which could host such a thing, which I don't have. Do you know what the "s3://nyqpug" bucket is? For the unit tests, it would be helpful to be able to host gzip and bzip2 files there as well.
 I have a pandas bucket where can put stuff
push the files up (to your branch and give me a pointer and I will put them up)
 Thanks! I'll ping you when that's done.
 Test using chunksize added.
 @jreback , I created compressed versions of the "tips.csv" table for unit tests and large tables of random data for performance regression tests. All are in the commit at https://github.com/stephen-hoover/pandas/commit/36b5d3a98efb4a0365dae6cc6883aea901143069 .
 all uploaded here (its a public bucket) `https://s3.amazonaws.com/pandas-test/`

though let's make the big ones only in the perf tests and/or slow
 Thanks! Could you put the file from "s3://nyqpug/tips.csv" in that bucket as well? The tests will be simpler if everything's in the same place.
 done
 Are the files set publicly readable? I can see the contents of the bucket, but when I try to read a file, I get a "Forbidden" error. That happens with the aws CLI as well as with `read_csv`.
 should be fixed now. annoying that I had to do that for each file :< individually
 Thanks. The S3 tests are more extensive now. I'll start working on performance tests, but I won't be able to have those until tomorrow afternoon.

I'll want to either rebase this PR on a merged #11072 or vice-versa. After that PR merges, the (Python 3) C parser will be able to read bz2 files from S3, and I can change the new tests to reflect that.
 With #11072 merged in, I've updated the tests to reflect the fact that the Python 3 C parser can now read bz2 files from S3. I've also added a set of benchmarks for reading from S3 in the `asv_bench/benchmarks/io_bench.py` file. The benchmarks read 10 rows of the 100,000 row x 50 column table of random noise in the pandas-test bucket. On the current (9ef85345a0088660883535f06dcdccf241eae9b2) master branch, benchmarks are

```
Python 2:
               ============= ======== ========
               --                  engine     
               ------------- -----------------
                compression   python     c    
               ============= ======== ========
                    None      27.90s   27.46s 
                    gzip      13.20s   13.13s 
                    bz2       20.43s    n/a   
               ============= ======== ========

Python 3:
               ============= ======== ========
               --                  engine     
               ------------- -----------------
                compression   python     c    
               ============= ======== ========
                    None      27.16s   27.34s 
                    gzip      14.42s   13.82s 
                    bz2       11.69s   11.77s 
               ============= ======== ========
```

and on this PR, the benchmarks are

```
Python 2:
               ============= ========== ==========
               --                    engine       
               ------------- ---------------------
                compression    python       c     
               ============= ========== ==========
                    None      208.45ms   486.83ms 
                    gzip       12.79s     13.10s  
                    bz2        20.39s      n/a    
               ============= ========== ==========

Python 3:
               ============= ========== ==========
               --                    engine       
               ------------- ---------------------
                compression    python       c     
               ============= ========== ==========
                    None      207.24ms   484.90ms 
                    gzip      249.83ms   363.71ms 
                    bz2       608.92ms   592.51ms 
               ============= ========== ==========
```

Note that in the > 1 s benchmarks, all of the time is taken in downloading the entire file from S3. Times will vary with network speed. It should be obvious if a future change forces `read_csv` to ingest the entire file from S3 again.

ASV is a really keen tool. I'm happy I found it.
 ok, this seems reasonable. pls add a note in whatsnew. ping when all green.
 @jreback , green!
 can you rebase?
 thank you sir!
