This closes a race condition for having the daemon idle exit.  After
the daemon has released its bus name, the dbus-daemon will no longer
allow messages through that targeted its unique name.

Since the intention of the `RegisterClient` method is to be the "knock on the
door", fix this by directly sending a message to the well-known name.

Second, we need to handle the case where the daemon exits without
replying; @jlebon added a `sleep(10)` invocation after the daemon
mainloop quit but before we `ReleaseName`, and I verified these two
things combine to fix that case.

Now that we have the ability to both track clients and our active transaction,
and the `RegisterClient` call acts "atomically", let's start doing exit-on-idle
and return the RAM to the people.