Decouple tendermint-rpc from tendermint-proto

[mzabaluev]

Description

After #1203 and #1090, a few direct uses of tendermint-proto remain in tendermint-rpc, so I see an opportunity to remove the direct API link in preparation to #1193. The only significant change is to replace direct use of a generated TxProto struct with a domain type. That type will also use Hash in all proper places.

Definition of "done"

tendermint-rpc no longer lists tendermint-proto in its dependencies, its public API only exposes tendermint types and functions (some of which may be re-imports of tendermint-proto).