Fix race in unix socket code when closing a socket that has

already sent a recv/send event.

When doing isc_socket_cancel we need to purge the event that might
already be in flight. If it has been launched already we need
to inform it that it has to bail.
42 jobs for !1915 with 834-fix-races-in-socket-code-v2 in 10 minutes and 26 seconds (queued for 5 seconds)
merge request