bpo-41486: Faster bz2/lzma/zlib via new output buffering (GH-21740)

Faster bz2/lzma/zlib via new output buffering.
Also adds .readall() function to _compression.DecompressReader class
to take best advantage of this in the consume-all-output at once scenario.

Often a 5-20% speedup in common scenarios due to less data copying.

Contributed by Ma Lin.