event: follow up to r1762701.

event: follow up to r1762701.
Keep QUEUE_APPEND()+pollset_add() or QUEUE_REMOVE()+pollset_remove() atomic.

Otherwise when a worker adds an entry in some queue (e.g. KA, lingering), it
might race with the listener in the time between the mutex is released and the
pollset is updated; meanwhile the listener might process the queue and find an
entry no yet in its pollset.

For the lingering queue, the entry could then have been used after its pool
destroyed.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1779354 13f79535-47bb-0310-9956-ffa450edef68