mod_proxy: fix proxy connection cleanup from an n+2 pool.

When connection_destructor() is called when pconf is gone, we can't dereference
worker->cp anymore. This happens is one_process mode exit if we apr_terminate()
or destroy the process pool directly (APR_POOL_DEBUG is needed too).

Fix this by NULL-ing worker->cp in conn_pool_cleanup(), and by registering it
as a pre_cleanup.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1822531 13f79535-47bb-0310-9956-ffa450edef68