Refactor StringValueStore to use external memory to limit memory usage.

[narekgharibyan]

JsonValueStore already has this functionality implemented. Probably it's possible to factor out common code and reuse it in both value stores.

[hendrikmuhs]

An option I see is using template specialization (or class inheritance?): The StringValueStore and the JsonValueStore should only differ by 2 operations: encoding and decoding json, everything else should be almost identical.

[narekgharibyan]

To me it sounds like a good application for inheritance, but @amit-cliqz is going to investigate this on upcoming days.

[hendrikmuhs]

While working on #93, I realized that the StringValueStore is quite different to the JsonValueStore, so reusage is more complicated. #93 is a step forward, it reuses parts and makes use of external memory.

Some background:

The JsonValueStore has been based on StringValueStore but later been completely rewritten to support compression. As this has been a breaking change JsonValueStore got renamed to JsonValueStoreDeprecated and the new JsonValueStore took it's place using a new enum value. For a while both existed while JsonValueStoreDeprecated got faded out and finally removed somewhen in 0.1 days, only the enum is still there.

I would actually do the same for StringValueStore in order to support compression, this new StringValueStore will - as discussed - share code with JsonValueStore. The old StringValueStore will be renamed and finally removed in 0.4 or 0.5.

[narekgharibyan]

As #93 also implements the functionality this issue requires, I'd suggest to close this one with #93 merge, and later on open a new issue to "add a compression functionality and make use of common code".

[hendrikmuhs]

fixed by #93