glusterd/snapshot: Putting back the missed_snaps_list code

Setting of missed_snap_count was removed as part of an
earlier patch. Putting back the code.
Signed-off-by: Avra Sengupta <asengupt@redhat.com>

Change-Id: Ib6412d6100145e94d10f6f4a8a1fe4e645c1a69e
BUG: 1097725
Reviewed-on: http://review.gluster.org/7764
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>