Wait for IRC listener cleanup and HTTP shutdown on stop (closes #159)

Stopping the server returned before the IRC listener's connections had
run their cleanup, and the database closed under them, so their
sessions, clients and channel memberships stayed and their nicks stayed
taken after a restart.

The IRC listener's stop now waits up to five seconds for every
connection's cleanup, which now runs on a context that stopping does
not cancel. The HTTP server's stop hook now runs Shutdown itself and
waits for it; httpServer is set before serving starts, and the server's
own signal watcher, which raced fx's stop hooks, is gone.

Model: opus-5-5

internal/ircserver/export_test.go

@@ -85,6 +85,10 @@ const PingInterval = pingInterval
 // for a line before it closes the connection.
 const PongDeadline = pongDeadline
 
+// StopTimeout is the longest Stop waits for the connections'
+// cleanup.
+const StopTimeout = stopTimeout
+
 // NewTestConn returns a Conn for nick on tcpConn, without a
 // server, database or service behind it.
 func NewTestConn(