Stop the slow test packages waiting (closes #263)

- Tests hash passwords at bcrypt.MinCost through db.PasswordCost.
- Test IRC servers check the queue only on broker notification;
  one test keeps the 100 ms fallback poll.
- The stop-timeout test shortens the timeout through a test-only
  setter.
- service and handlers tests build config.Config directly instead
  of through config.New, so they run in parallel: service had run
  serially, and handlers started its servers one at a time.
- Repeated race trials run as parallel subtests.
- Fixed sleeps become waits on the broker.
- A new test covers the relay's failed queue read, which only the
  100 ms poll had reached.

Model: opus-5-5

internal/ircserver/relay.go

@@ -17,7 +17,7 @@ import (
 func (c *Conn) relayMessages(ctx context.Context) {
 	// Use a ticker as a fallback; primary wakeup is via
 	// broker notification.
-	ticker := time.NewTicker(pollInterval)
+	ticker := time.NewTicker(c.pollInterval)
 	defer ticker.Stop()
 
 	for {