Will need to do subclass-specific implementations in the next commit.
This isn't because there will be multiple variations of the daemon
protocol (whew!) but because different clients pick and choose different
parts to use.
Pass this around instead of `Source &` and `Sink &` directly. This will
give us something to put the protocol version on once the time comes.
To do this ergonomically, we need to expose `RemoteStore::Connection`,
so do that too. Give it some more API docs while we are at it.
