Remove the relay's broker wait channel after each tick (closes #151)

The IRC relay registers a broker wait channel on each pass of its
loop, and when the 100 ms ticker fired instead of a notification it
looped without removing that channel. An idle IRC listener connection
added ten channels a second to the broker, and all but the last stayed
after the connection closed. The ticker case now removes its channel,
as the HTTP long-poll does after its timer.

Broker.ListenerCount returns how many wait channels a user holds, so
the test can check that an idle connection holds at most one, and
none after it closes.

Model: opus-5-5

internal/ircserver/export_test.go

@@ -60,6 +60,10 @@ func (s *Server) Listener() net.Listener {
 // to a killed victim.
 const KillWriteWindow = killWriteWindow
 
+// PollInterval is how often the relay checks the client
+// output queue when no broker notification arrives.
+const PollInterval = pollInterval
+
 // NewTestConn returns a Conn for nick on tcpConn, without a
 // server, database or service behind it.
 func NewTestConn(