snaptree-based in-memory storage

Until a full-fledged MV store is implemented we can implement in-memory storage on a snaptree library [1] that represents a concurrent AVL tree with support of snapshots.

In this ticket we need to integrate the library with our existing storage APIs (refine API if necessary), integrate its snapshot API with Raft snapshots and provide configuration if necessary.

[1] https://github.com/nbronson/snaptree