cluster/afr: Provide an option to disable afr durability

Change-Id: I40eec20ca6b3f857245a2438883822e251077ee9
BUG: 979365
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/5269
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>