Merge r1572630, r1572611, r1572967, r1573229 from trunk:

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.


Follows up r1572606.
MMN minor bump required by proxy_conn_rec change.


mod_proxy: follows up r1572630.
Don't reuse a SSL backend connection with no SNI for a request requiring SNI.


mod_proxy: Add comment and avoid ternary operator as condition (no functional change).
Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1587201 13f79535-47bb-0310-9956-ffa450edef68