geo-rep: History Change detector method select issue

Geo-rep does history crawl even if change-detector is set
to xsync.

This patch fixes by taking priority to user configured
change detector.

BUG: 1113525
Change-Id: Ic6c34e187c9cb6608c9ef8a010ea07015ba60a80
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://review.gluster.org/8183
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kotresh HR <khiremat@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Venky Shankar <vshankar@redhat.com>