PERF: Checking monotonic-ness before sorting on an index #11080
closes #11080 

Worked for this during PyCon JP 2015 pandas sprint /w @sinhrks 

I found `test_frame.py:TestDataFrame.test_sort_index` cover this change.

pls add some benchmarks that I indicated in the issue (or point to ones that have this covered)
 Added a `frame_sort_index` test in `frame_methods.py`
-   30.96ms     4.64ms      0.15  frame_methods.frame_sort_index.time_frame_sort_index
 @scari I think you did something wrong with rebasing (as there are a lot of other commits here). Can yo do

```
git fetch upstream
git rebase upstream/master
```

and then push again.
 @jorisvandenbossche  Thanks for your kind guidance! I just push again. 
 thanks @scari 
