Move SSL session data serialization up out of the session cache

Move SSL session data serialization up out of the session cache
storage providers:

* modules/ssl/ssl_private.h (modssl_sesscache_provider): Change
  'store' interface to take a data/length pair rather than an
  SSL_SESSION pointer.

* modules/ssl/ssl_scache.c (ssl_scache_store): Serialize the SSL
  session here and pass down the raw DER.

* modules/ssl/ssl_scache_dc.c, modules/ssl_scache_mc.c,
  modules/ssl_scache_shmcb.c, modules/ssl_scache_dbm.c: Adjust ->store
  implementations accordingly, removing the four sets of identical
  code doing the i2d dance.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@630168 13f79535-47bb-0310-9956-ffa450edef68