This will allow us to factor out logic, which is currently scattered
inline, into several reusable instances
The tests are also updated to support versioning. Currently all Worker
and Serve protocol tests are using the minimum version, since no
version-specific serialisers have been created yet. But in subsequent
commits when that changes, we will test individual versions to ensure
complete coverage.
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.
