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>

init/tmpfilesd/ipa.conf.in

@@ -1,4 +1,3 @@
-d @localstatedir@/run/ipa_memcached 0700 apache apache
 d @localstatedir@/run/ipa 0700 root root
 d @localstatedir@/run/httpd/ipa 0700 apache apache
 d @localstatedir@/run/httpd/ipa/clientcaches 0700 apache apache