Query trait

[Matt Bell]

Stores should be able to implement a Query trait so that clients can ask for N keys or ranges of keys and receive their values. It will look something like this:

trait Query {
  fn query(key: &[u8]) -> Result<Vec<u8>>;
}

In MerkStore the query implementation will return a Merkle proof, in WriteCache/MapStore the implementation will basically just be a simple get.

For now, we don't need to support ranges of keys since that isn't available in the Read trait yet. Also, for now, the query can just be a single key but could later be an array of keys. It's also possible that we'll just always keep the "key" argument as a single byte slice which can be deserialized differently by different stores.

[Matt Bell]

We'll also want to require this for the store used in the ABCI state machine, so that it can respond to ABCI query messages with the store's query implementation.