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