Switch to gRPC API

[cpacia]

gRPC provides a much nicer interface than the JSON RPC and websocket APIs. There is a WIP PR in btcd to add gRPC support btcsuite/btcd#1075.

We could build on that and get it to where it needs to be to merge in here. Once that's in I would prefer to remove all JSON RPC API altogether and get rid of the btcjson package.

gRPC has a nice feature in that there is a plugin that lets you spawn a REST reverse proxy to the API for people who absolutely need a REST JSON interface.

In the course of doing so we should make sure we add all the RPCs needed to use bchd as a backend SPV wallet server. This would imply a creating a getheaders endpoint which accepts a block locator just like the wire message does. Plus a streaming headers endpoint for wallets to subscribe to.

Finally we'll need to serve SPV proofs either from a separate API or with the transactions response.

[swdee]

For reference SPV proof ability is available with RPC commands gettxoutproof and verifytxoutproof implemented in this PR #153