event: Use skiplist and fold in performance tweeks from trunk

trunk patch: http://svn.apache.org/viewvc?view=revision&revision=1451706
             http://svn.apache.org/viewvc?view=revision&revision=1517365
             http://svn.apache.org/viewvc?view=revision&revision=1529442
2.4.x patch: http://people.apache.org/~jim/patches/httpd-2.4-event-v3.patch

Submitted by: jim
Reviewed by: rjung, minfrin



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1542560 13f79535-47bb-0310-9956-ffa450edef68