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.