Make the callback for the hello command in SingleServerDiscoveryMonitor not take the mutex if the command was canceled due to shut down

Currently, the callback for the hello command sent by the SingleServerDiscoveryMonitor takes the mutex before checking for shut down. We could let a thread lock in the case where the ReplicaSetMonitor task executor thread acquires SingleServerDiscoveryMonitor's mutex when calling SingleServerDiscoveryMonitor::shutdown and then tries to acquire it again when calling the callback function after cancelling pending a hello command. To avoid this, the callback should not take the mutex if the status is CallbackCanceled.