event: add/remove from/to the pollset outside of the critical sections.

We don't need external locking since it's created with APR_POLLSET_THREADSAFE,
hence reduce those sections to the lowest cycles possible.

A spinlock may be interesting instead of the mutex now, we won't block and the
TO_QUEUE_*() and process_timeout_queue() operations are fast...



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1762701 13f79535-47bb-0310-9956-ffa450edef68