Refactor interfaces exposed by for persistence layer to make them more useful for ABCI apps

[adrianbrink]

• [ ] expose go interfaces for the persistence layer
• [ ] C wrapper around the go interfaces to enable users to import a persistence layer into any ABCI app
• [ ] socket protocol around persistence interfaces

https://github.com/tendermint/tendermint/blob/master/docs/architecture/merkle-frey.md

@ethanfrey Please add your ideas here as well.

[ethanfrey]

This should expose more of the functionality of IAVLTree....

KVStore: Get Set List (in range with start, end, limit) Remove

Merkle: Hash (calculate root hash) Proof (for Get or List with root hash) Copy/Commit (or something else to make temporary transaction, we can choose to use or throw away)