mod_ssl: Add SSLSessionTickets (on|off).
It controls the use of TLS session tickets (RFC 5077).
Default is unchanged (on).
Using session tickets without restarting the web server with
an appropriate frequency (e.g. daily) compromises perfect forward
secrecy. As long as we do not have a nice key management
there needs to be a way to deactivate the use of session tickets.

Submitted by: rjung
Reviewed by: rjung, covener, ylavic
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1652074 13f79535-47bb-0310-9956-ffa450edef68