Rework the btcd RPC connection.

This change greatly cleans up the RPC connection between btcwallet and
btcd.  Proper (JSON-RPC spec-following) notifications are now expected
rather than Responses with a non-empty IDs.

A new RPCConn interface type has also been introduced with a
BtcdRPCConn concrete type for btcd RPC connections.  Non-btcd-specific
code handles the RPCConn, while the btcd details have been abstracted
away to a handful of functions.  This will make it easier to write
tests by creating a new fake RPC connection with hardcoded expected
replies.