Added mock data storage and a test for it.

Hi Brian,

Below is the idea of how I see the storage and a test for it.
The test is templated to support LevelDB interface.

The logic of having the streams append-only and not overwriting past data, as well as creating binary forms of keys and the secondary ones will be in another module. I plan to call it ```stream_storage```, to complement ```data_storage```.

This pull request is one commit:

https://github.com/dkorolev/tailproduce/commit/8805844ae329664d8af1348f62018aed23702ed4

if https://github.com/Staance/tailproduce/pull/6 is merged.

Thanks,
Dima