Summary:
Currently, we don't use options.compression_per_level[0] as the compression style for L0 compression type, unless it is None. This behavior
 doesn't look like on purpose. This diff will make sure L0 compress using the style of options.compression_per_level[0].

Reviewed and accepted in: https://reviews.facebook.net/D65607
Closes https://github.com/facebook/rocksdb/pull/1435

Differential Revision: D4099368

Pulled By: siying

fbshipit-source-id: cfbbdcd