Re-implement the SSL session cache abstraction using a vtable; first

Re-implement the SSL session cache abstraction using a vtable; first
step towards use of the ap_provider interface:

* modules/ssl/ssl_private.h (modssl_sesscache_provider): Add new
  vtable type.
  (SSLModConfigRec): Reference the vtable here.
  Replace all the ssl_scache_* prototypes with provider vtable objects.

* modules/ssl/ssl_scache.c (ssl_scache_init, ssl_scache_kill,
  ssl_scache_retrieve, ssl_scache_store, ssl_scache_remove,
  ssl_ext_status_hook): Use callbacks from vtable rather than ifdef
  spaghetti.

* modules/ssl/ssl_engine_init.c (ssl_init_ctx_session_cache):
  Only install the OpenSSL callbacks if a vtable is configured.

* modules/ssl/ssl_engine_config.c (ssl_cmd_SSLSessionCache): Set up
  vtable pointer.

* modules/ssl/ssl_scache_dc.c, modules/ssl_scache_mc.c: Adjust to make
  implementations static, and add vtable definition.

* modules/ssl_scache_shmcb.c: Likewise; also move the init
  one-per-process requirement down here.

* modules/ssl_scache_dbm.c: Likewise; also (temporarily) use a local
  subpool in the store callback.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@630163 13f79535-47bb-0310-9956-ffa450edef68