Add Range modifier for abci query

[ethanfrey]

Currently we allow queries for one key or by prefix. Prefix queries use the Iterator function underneath, but only support a subset of possible queries, as it was simpler to define the interface at abci level. Let us expose this power.

• define the arguments to range modifier (start, end, reverse?? More??)
• define struct to be posted. Json? Protobuf? This goes into the binary data field for the abci query
• add support for this struct with a new range modifier in orm.Bucket.Query
• parse this struct in app.StoreApp.Query
• update the AppHelper from the benchmark tests to make use of these query types to expose full ReadOnlyKVStore interface over abci.

And of course test this all