afr: customize client-pid=-1 xtime aggregation to tolerate a replica down

Using the new 'pluggable policies' API of libxlator.

Change-Id: Ie7528182dff8fb42c6e8287a106d3057944df775
BUG: 847839
Original Author: Csaba Henk <csaba@redhat.com>
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/4904
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>