use ap_varbuf instead of allocating new strings each time we override an old one.

This uses leaks less memory, but it's still not perfect (but it's a start - maybe I need to use a mutex for this, to override the original object without running into race conditions)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1470155 13f79535-47bb-0310-9956-ffa450edef68