Get the current time only once and pass it around so that sessions don't have to
get the current time anymore.
Add experimental support for a GSource that dispatches when the session needs to
be cleaned up.
Add the timeout value to the Session header for unusual timeout values.
Allow us to configure a limit to the amount of active sessions in a pool. Set a
limit on the amount of retry we do after a sessionid collision.
Add properties to the sessionid and the timeout of a session. Keep track of
creation time and last access time for sessions.
