Fix case where if the backup slurmdbd has existing connections when it gives...

Fix case where if the backup slurmdbd has existing connections when it gives up control that the it would be killed.

If the backup had existing connections when giving up control, it would try to
signal the existing threads by using pthread_kill to send SIGKILL to the
threads. The problem is that SIGKILL doesn't go the thread but the main process
and the backup dbd would be killed.