mod_proxy: pconf vs pchild consistency, and correctness in ONE_PROCESS mode.

mod_proxy: pconf vs pchild consistency, and correctness in ONE_PROCESS mode.

Consistently use pconf for ap_proxy_define_{worker,balancer}() and pchild for
ap_proxu_initialize_{worker,balancer}() in mod_proxy [child_]init code.

pchild is needed in _initialize() for mutexes/shms' child_init and cleanup,
and to avoid a crash on shutdown (i.e. ap_terminate) in ONE_PROCESS mode,
where worker->cp->pool is destroyed twice, let's register conn_pool_cleanup()
as a pre_cleanup of pchild.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1883744 13f79535-47bb-0310-9956-ffa450edef68