Fix a race in isc_socket destruction.

Fix a race in isc_socket destruction.

There was a very slim chance of a race between isc_socket_detach and
process_fd: isc_socket_detach decrements references to 0, and before it
calls destroy gets preempted. Second thread calls process_fd, increments
socket references temporarily to 1, and then gets preempted, first thread
then hits assertion in destroy() as the reference counter is now 1 and
not 0.