Implement session activity timeout

Previously sessions expired after session_auth_duration had elapsed
commencing from the start of the session. We new support a "rolling"
expiration where the expiration is advanced by session_auth_duration
everytime the session is accessed, this is equivalent to a inactivity
timeout. The expiration is still constrained by the credential
expiration in all cases. The session expiration behavior is
configurable based on the session_auth_duration_type.

* Reduced the default session_auth_duration from 1 hour to 20 minutes.

* Replaced the sesssion write_timestamp with the access_timestamp and
  update the access_timestamp whenever the session data is created,
  retrieved, or written.

* Modify set_session_expiration_time to handle both an inactivity
  timeout and a fixed duration.

* Introduce  KerberosSession as a mixin class to share session
  duration functionality with all classes manipulating session data
  with Kerberos auth. This is both the non-RPC login class and the RPC
  classes.

* Update make-lint to handle new classes.

* Added session_auth_duration_type config item.

* Updated default.conf.5 man page for new session_auth_duration_type item.

* Removed these unused config items: mount_xmlserver,
  mount_jsonserver, webui_assets_dir

https://fedorahosted.org/freeipa/ticket/2392

ipalib/krb_utils.py

@@ -388,5 +388,5 @@ class KRB5_CCache(object):
         except KeyError:
             pass
 
-        self.debug('"%s" ccache endtime=%s', self.ccache_str(), krb5_format_time(result))
+        self.debug('"%s" ccache endtime=%s (%s)', self.ccache_str(), result, krb5_format_time(result))
         return result