Zstd compression

[hendrikmuhs]

add zstd compression support

[narekgharibyan]

lgtm, just seems that the newly written files will be possible to open with new versions of keyvi only ... don't remember do we have some sort of min version requirement somewhere in code ?

[hendrikmuhs]

Good point! Yes, we have versioning in keyvi files. I will make use of it. It should be possible to only bump the version if zstd is used.

Currently an old version would throw an exception the 1st time a value has to be unpacked. With versioning in place it will not be possible to open a dictionary that uses zstd in an old version.

[hendrikmuhs]

added versioning, zstd now requires a recent keyvi version, trying to load a zstd dict in 0.6.3 compared to one that uses zlib compression (both dicts built using this PR):

>>> d=keyvi.dictionary.Dictionary("zstd.kv")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "_core.pyx", line 316, in _core.Dictionary.__init__
  File "_core.pyx", line 291, in _core.Dictionary._init_0
ValueError: this version of keyvi file is unsupported
>>> d=keyvi.dictionary.Dictionary("zlib.kv")
>>>