mod_ssl: Calculate the MD5 digest used as the session context once per

mod_ssl: Calculate the MD5 digest used as the session context once per
vhost at startup, rather than building it for each new connection.

* modules/ssl/ssl_private.h (struct SSLSrvConfigRec):
  Replace vhost_id_len field with vhost_md5.

* modules/ssl/ssl_engine_init.c (ssl_init_Module): Build the
  sc->vhost_md5 hash here.

* modules/ssl/mod_ssl.c: Fail at compile time if the
  SSL_set_session_id_context() API constraint on context length is
  violated.
  (ssl_init_ssl_connection): Use sc->vhost_md5.

* modules/ssl/ssl_engine_kernel.c (ssl_find_vhost): Use sc->vhost_md5
  after renegotiation.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1877349 13f79535-47bb-0310-9956-ffa450edef68