Modified rpc package to use message package.
Modified rpc package to use message package.

We were replicating some logic in the rpc package that already existed
in the message package. Unfortunately we can't replace rpc streams
entirely with message streams since the former also transmits error
information in message headers, but we can build rpc message streams on
top of normal message streams.

havoc-io/mutagen,havoc-io/mutagen