rbd-mirror: group-replayer check for remote demote state

rbd-mirror: group-replayer check for remote demote state

I'm seeing a possibility for 3 situations here for resync flagging and
rbd-mirror daemon working on it:

1. No Demotion on Primary while/just-before resync is play'ed
    there is no demote snap along side resync, we can cancel syncing other
    snaps, and start resync as soon as resync is flagged, because there is
    no point syncing snaps that we are anyway going to delete the whole
    group and resync fresh.

2. first Demote + immediately Resync
    demote came first, this mean before proceeding with resync, we should
    always see if the last remote snap is PRIMARY (validate if the remote
    is still primary, which is on point) and only proceed

3. first Resync + immediately Demote
    resync Came first, so we head straight to resync.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>