gsyncd / geo-rep: periodically set slave xtime on slave

setting the slave xtime on the slave (after each changelog/xsync)
crawl helps in two things:
  * effective recover of master (failover/failback)
  * cascading setup - instances when the session from intermediate
                      master session is stopped, data is put on
                      the master -> slave sesssion and then the
                      cascading session is started again.

Change-Id: Ifae10a6ac09dc0d17707c3b5a3090bcf1efec8b6
BUG: 990900
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/5451
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>