Destroy pmain pool when exiting ap_mpm_run() so that cleanups

registered in modules' child_init hooks are performed (e.g.,
mod_log_config and mod_dbd).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@491907 13f79535-47bb-0310-9956-ffa450edef68