cluster/afr: Add largest file is source policy

For Write Once Read Many times type of work-load choosing largest
file to be the source will always resolve fool-fool
scenarios correctly. In other cases we fsync() the files and
will have a reliable 'wise man'.

Change-Id: Ic4dbea8d06db6d578fbcb866fb65ee2d066ac7ba
BUG: 958118
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5519
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>