Merge r1588806 from trunk:

r1494157 from eventopt for event, per sf's review comments.

use a subpool of pchild since skiplist allocations will
happen across threads and are only protected from
other skiplist operations.




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1835179 13f79535-47bb-0310-9956-ffa450edef68