[AMQ-7452] Fix "Timer already cancelled" on new connections when shared inactivity timer dies

[jbonofre]

The static READ_CHECK_TIMER and WRITE_CHECK_TIMER in AbstractInactivityMonitor are shared across all transport instances. Java's Timer permanently marks itself cancelled when its internal thread exits due to an unhandled Error thrown by a TimerTask. SchedulerTimerTask rethrows Error subclasses, so an OutOfMemoryError in any timer task kills the shared timer thread while the static field remains non-null. All subsequent schedule() calls on new connections then throw IllegalStateException ("Timer already cancelled"), which propagates to TransportConnector.onAcceptError() and is logged as WARN "Could not accept connection: Timer already cancelled."

Here's my fix proposal: wrap the schedule() calls in startConnectCheckTask() and startMonitorThreads() in try/catch(IllegalStateException). On catch, the dead timer is cancelled (to let its thread exit), replaced with a fresh instance, and the task is rescheduled. The guard condition in startConnectCheckTask() is also extended with || READ_CHECK_TIMER == null to cover the case where the field is unexpectedly null when CHECKER_COUNTER > 0.

I added three regression tests to InactivityMonitorTest covering:

• READ_CHECK_TIMER cancelled with CHECKER_COUNTER == 0 (startConnectCheckTask path)
• READ_CHECK_TIMER cancelled with CHECKER_COUNTER > 0 (production scenario)
• WRITE_CHECK_TIMER cancelled (startMonitorThreads path)

Also, tests use Wait.waitFor() instead of Thread.sleep() for reliability on slow CI.