[Breaking] Switch text loading to multi-threaded PyArrow loading
Awesome !
Also I was wondering if we should try to make the hashing of the `data_files` faster (it is used to build the cache directory of datasets like `text` or `json`). Right now it reads each file and hashes all of its data. We could simply hash the path and some metadata including the `time last modified` tag no ? Apparently we can get this tag with `os.path.getmtime(path)` I just rebased from master to include the hashing changes from #573  I think this is ready to merge, no? Indeed it's ready to merge :) Ok added the breaking change info and we can merge indeed.

Test if we can get better performances for large-scale text datasets by using multi-threaded text file loading based on Apache Arrow multi-threaded CSV loader.

If it works ok, it would fix #546.

**Breaking change**:
The text lines now do not include final line-breaks anymore.