The current implementation of subscribe is difficult to mock because
the queue that we send updates on in unexported, so you cannot create
a subscribe.Client object and then add your own updates. While it is
possible to run a subscribe server in tests, subscribe servers will
shutdown before dispatching their udpates to all clients, which can be
flakey (and is difficult to workaround). In this commit, we add a
subscription interface so that these testing struggles can be addressed
with a mock.
