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.

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")
>>>