[Breaking] Switch text loading to multi-threaded PyArrow loading
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['Awesome !\r\nAlso 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.\r\n']
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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.
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