mpm_event, mpm_worker: Remain active amidst prevalent child process

resource shortages.

Submitted by: trawick
Reviewed by: jorton, rjung, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1365003 13f79535-47bb-0310-9956-ffa450edef68