mod_proxy: Shutdown (eg. SSL close notify) the backend connection
before closing.


mod_proxy: follow up to r1601291.

Since deferred_write_pool is needed by the core_output_filter and is a subpool
of the connection, shutdown in a pre_cleanup of the connection's pool to avoid
a freed memory access (SEGV).

Reported By: takashi

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1651080 13f79535-47bb-0310-9956-ffa450edef68