Allow loading dataset in-memory
Allow loading datasets either from:
- memory-mapped file (current implementation)
- from file descriptor, copying data to physical memory

Close #708
I am wondering how to test their difference... > ring how to test their difference...

Hmm I don't think pyarrow exposes an API to check if a Table comes from a file that is memory-mapped. In particular since all the buffer/memory logic is in the C++ part of pyarrow.

Otherwise we can still check the difference of RAM used when loading a big chunk of data. > Hmm I don't think pyarrow exposes an API to check if a Table comes from a file that is memory-mapped. In particular since all the buffer/memory logic is in the C++ part of pyarrow.
> 
> Otherwise we can still check the difference of RAM used when loading a big chunk of data.

@lhoestq I think I found a way: `pa.total_allocated_bytes()` :smirk: