Merge r1666468, r1666618, r1674697 from trunk:

mpm_event: Allow for timer events duplicates. 
Meanwhile ap[r]_skiplist_add()...


mpm_event: follow up to r1666468.
We only need one compare function for add semantic with apr_skiplist_insert()
and unique timers (pointers). It also should work with apr_skiplist_remove()
and apr_skiplist_find(), be they used some day.


mpm_event: follow up to r1666468 and r1666618.
We don't need to return 0 in the compare function, but for debugging purpose
which we could implement later if necessary (in a separate function).
For now, keep the function simple as in 2.4.x to ease backport, and add a
comment about why we never return 0 here.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1674921 13f79535-47bb-0310-9956-ffa450edef68