backport r1213380 from trunk:

Streamline TLS session ticket key handling (added in r1200040):
- drop the SSLTicketKeyDefault directive, and only support a single
  ticket key per server/vhost
- rename the SSLTicketKeyFile directive to SSLSessionTicketKeyFile,
  remove the keyname parameter
- move ticket key parameters from SSLSrvConfigRec to modssl_ctx_t
- configure the tlsext_ticket_key_cb only when in server mode
- add documentation for SSLSessionTicketKeyFile


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1213395 13f79535-47bb-0310-9956-ffa450edef68