Forwarded request #1099794 from ateixeira

- Update to 0.15.9
- ZstdFile class related changes:
* Add SeekableZstdFile class, it's a subclass of ZstdFile, supports
Zstandard Seekable Format.
* Add mode argument to ZstdFile.flush() method, now it can flush
a zstd frame.
* Add read_size and write_size arguments to ZstdFile.__init__()
method, can work with Network File Systems better.
* Optimize ZstdFile performance to C language level.
- Changes from 0.15.7
- ZstdDict class changes:
* Fix these advanced compression parameters may be ignored when
loading a dictionary: windowLog, hashLog, chainLog, searchLog,
minMatch, targetLength, strategy, enableLongDistanceMatching,
ldmHashLog, ldmMinMatch, ldmBucketSizeLog, ldmHashRateLog,
and some non-public parameters.
* When compressing, load undigested dictionary instead of digested
dictionary by default. Loading again an undigested is slower, see
differences.
* Add .as_prefix attribute. Can use zstd as a patching engine.