Investigate VInt performance improvements

[hendrikmuhs]

Compile time: We use variable length integers and shorts quite heavily, an improvement on the encoding (maybe also for decoding == lookup) might have an impact.

File: https://github.com/KeyviDev/keyvi/blob/master/keyvi/include/keyvi/util/vint.h

Idea: Loop unrolling (inspired by https://github.com/lemire/FastPFor) might speedup things.

Worth a try, should be (micro-)benchmarked.