mod_proxy/ssl: cleanup per-request SSL configuration for recycled proxy conns.

The SSL dir config of proxy/backend connections is stored in r->per_dir_config
but those connections have a lifetime independent of the requests they handle.

So we need to allow the external ssl_engine_set() function to reset mod_ssl's
dir config in between proxy requests, or the first sslconn->dc could be used
after free for the next requests.

mod_proxy can then reset/reinit the request config when recycling its backend
connections.

* Solve a chicken and egg problem here:
  We need to have sslconn->dc set correctly when we want to
  init sslconn, but we need to allocate memory for it first.

PR 63256.
Submitted by: ylavic, rpluem
Reviewed by: ylavic, jorton, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1855918 13f79535-47bb-0310-9956-ffa450edef68