Change session handling
    
    Stop using memcache, use mod_auth_gssapi filesystem based ccaches.
    Remove custom session handling, use mod_auth_gssapi and mod_session to
    establish and keep a session cookie.
    Add loopback to mod_auth_gssapi to do form absed auth and pass back a
    valid session cookie.
    And now that we do not remove ccaches files to move them to the
    memcache, we can avoid the risk of pollutting the filesystem by keeping
    a common ccache file for all instances of the same user.
    
    https://fedorahosted.org/freeipa/ticket/5959
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>