Store "Integer strings" as Integers

[helje5]

The current implementation stores "integer strings" as ByteBuffers. A key is turned into an "integer string" when the INCR, DECR family of commands are used.

SET counter 22
INCRBY counter 20

This is currently slower than necessary, we parse the ByteBuffer's into Integers, do the op, and persist as ByteBuffers again.

The RedisValue enum is prepared for that. It isn't a hard change, but will affect many places.