Merge r1729826, r1729847, r1732986, r1733056 from trunk:

mod_proxy: Play/restore the TLS-SNI on new backend connections which
had to be issued because the remote closed the previous/reusable one
during idle (keep-alive) time.


mod_proxy: follow up to r1729826: really copy conn->ssl_hostname.

mod_proxy: follow up to r1729826 + r1729847.
Adjust stacked ssl_hostname maximum size.

mod_proxy: follow up to r1729826 + r1729847 + r1732986.
Don't use magic constants.
Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1733474 13f79535-47bb-0310-9956-ffa450edef68