use the new 'apr_hash_merge' function to do the overlays.

In our testing this resulting in a large performance win (>5%)

Submitted by:	 Brian Pane <bpane@pacbell.net>
Reviewed by:	Ian Holsman


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91825 13f79535-47bb-0310-9956-ffa450edef68