Redo what was reverted in r1572627.

Don't reuse a SSL backend connection whose SNI differs. PR 55782.
This may happen when ProxyPreserveHost is on and the proxy-worker
handles connections to different Hosts.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1572630 13f79535-47bb-0310-9956-ffa450edef68