gsyncd / geo-rep: introduce "recovery" mode

This mode introduces and optimized recovery of master,
used during failover-failback cases. Files are only
transferred if they are identified as changed on the slave.

A normal failback mechanism would be just to reverse the
direction of geo-replication syncing. Doing this is the
correct way but could be time consuming. "Recovery" mode
optimizes this by identifying files (as skipping them) duing
the one-shot crawl.

NOTE: Use this mode when you are _absolutely_ sure that the
namespace on the master is intact, ie. there is number of files
in master is equal to the number of files in the slave, but the
slave was written to (after a failover) when master was brought
back up.

- gluster volume geo <MV> <SLAVE>::<SV> config special_sync_mode recover
- gluster volume geo <MV> <SLAVE>::<SV> start

Change-Id: Idb698949e4e86698b15d04a35d33095562f51714
BUG: 994462
Original Author: Venky Shankar <vshankar@redhat.com>
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/5515
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>