crimson/osd: reject Seastore PG merges across shards

crimson/osd: reject Seastore PG merges across shards

Seastore cannot merge collections between reactor shards currently.
On cross-shard detection, tell the monitor the source PG is not ready
(via MOSDPGReadyToMerge{ ready=false }) so the unsafe pg_num decrement
is never proposed, then send MOSDPGStopMerge to clamp pg_num_target and
permanently disable further shrink for the pool.

Signed-off-by: Aishwarya Mathuria <amathuri@redhat.com>