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/conn.go

@@ -70,6 +70,10 @@ type Conn struct {
 	closed      bool
 	killed      bool
 
+	// pollInterval is how often the relay checks the client
+	// output queue when no broker notification arrives.
+	pollInterval time.Duration
+
 	// quitReason is the reason in the QUIT that peers are
 	// sent when the connection ends. The client's QUIT
 	// replaces it.
@@ -83,6 +87,7 @@ func newConn(
 	brk *broker.Broker,
 	cfg *config.Config,
 	svc *service.Service,
+	poll time.Duration,
 ) *Conn {
 	host, _, _ := net.SplitHostPort(tcpConn.RemoteAddr().String())
 
@@ -101,6 +106,8 @@ func newConn(
 		serverSfx:  srvName,
 		remoteIP:   host,
 		quitReason: "Connection closed",
+
+		pollInterval: poll,
 	}
 
 	conn.commands = conn.buildCommandMap()