glusterd/snapshot-handshake: Perform handshake of missed_snaps_list.

In a handshake, create a union of the missed_snap_lists of the two peers.
If an entry is present, its no op.
If an entry is pendng, and the peer entry is done, mark own entry as done.
If an entry is done, and the peer ertry is pending, its a no-op.
If its a new entry, add it.

Change-Id: Idbfa49cc34871631ba8c7c56d915666311024887
BUG: 1061685
Signed-off-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-on: http://review.gluster.org/7453
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>