stream-ssl: Set a session context ID string on our SSL_CTX.

stream-ssl: Set a session context ID string on our SSL_CTX.

In the finest OpenSSL tradition of putting important documentation only in
code comments, ssl/ssl_sess.c in the OpenSSL tree has the following comment
inside ssl_get_prev_session():

   /* We can't be sure if this session is being used out of
    * context, which is especially important for SSL_VERIFY_PEER.
    * The application should have used SSL[_CTX]_set_session_id_context.
    *
    * For this error case, we generate an error instead of treating
    * the event like a cache miss (otherwise it would be easy for
    * applications to effectively disable the session cache by
    * accident without anyone noticing).
    */

This meant that ovs-controller couldn't effectively cache SSL server
sessions and we got a weird error whenever ovs-vswitchd tried.

Bug #4448.
CC: David Tsai <dtsai@nicira.com>
CC: Jeremy Stribling <strib@nicira.com>