[FEATURE] Add gracetime for session-timestamp-updates

Don't update the session-timestamp more often than
needed. Skipping updates for some seconds/minutes
makes no difference but may save quite some
updates.

Change-Id: Ic812993f3c7556fc90a1a5dee7b5462e822d9f45
Resolves: #64148
Releases: master
Reviewed-on: http://review.typo3.org/35838


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>