Avoid spurious directory metedata split brain

When directory content is modified, [mc]time is updated. On
Linux, the filesystem does it, while at least on NetBSD, the
kernel file-system independant code does it. This means that
when entries are added while bricks are down, the kernel sends
a SETATTR [mc]time which will cause metadata split brain for
the directory. In this case, clear the split brain by finding
the source with the most recent modification date.

BUG: 1129939
Change-Id: Ic0177e0df753a4748624d0b906834ed54593adb9
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9291
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>