Implement new StringValueStore supporting compression

[hendrikmuhs]

Followup of #69

The StringValueStore should be re-implemented to reuse the same techniques as todays JsonValueStore which is supporting compression and using length-prefixes instead of zero-termination.

To ensure backwards compatibility rename StringValueStore to StringValueStoreDeprecated, add a new StringValueStore using a new enum value for the type. While writer and mergers can be deleted the reader must be kept to support existing keyvi files at least until the next bigger release.

The new StringValueStore and the JsonValueStore should only differ by 2 operations: encoding and decoding json, everything else should be almost identical.