NexusKVStore automated A/B verification of KVStore implementations

It would be good to create some additional framework around KVStore such that we could run two in parallel (of any type) and compare the results of any operation against one against the results against another. They should generally (always?) be the same. There are some interesting cases where things work differently between the current KVStores (such as compaction) that may prove less than trivial in doing so. Such a KVStore should be a drop in replacement for any other KVStore so that we could even run this in QE tests. This may required some refactoring such that we separate the API of KVStore from the common implementations that live there.

Brief overview on NexusKVStore - https://docs.google.com/document/d/1bbak-rTblXY8AjMYRsxcY5oJrhwMJs5y_61R8my4Mko/edit#