cluster/*: get logic to calculate min() of the 'stime' xattr

* in both distribute and replicate (ignoring stripe for now),
  add logic to calculate the min() of stime values.

* What is a 'stime' ? Why is this required:
  -  stime means 'slave xtime', mainly used to keep track of slave
  node's sync status when distributed geo-replication is used.
  Logic of calculating 'min()' for this stime is very important as
  in case of crashes/reboots/shutdown, we will have to 'restart'
  with crawling from stime time value from the mount point, which
  gives the 'min()' of all the bricks, which means, we don't miss
  syncing any files in the above cases.

Change-Id: I2be8d434326572be9d4986db665570a6181db1ee
BUG: 847839
Original Author: Amar Tumballi <amarts@redhat.com>
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4893
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>