Simple dout() fix

Signed-off-by: David Zafman <david.zafman@inktank.com>
Backfill peers should not be included in the acting set

Create actingbackfill in choose_acting()
Use first backfill target as previously
Add asserts to catch inappropriate use of actingbackfill

fixes: #5855

Signed-off-by: David Zafman <david.zafman@inktank.com>
Use is_acting() in proc_replica_info() because this is before actingbackfill set
We may need to recompute stray_set to prevent purge_strays from removing collection

Can't check is_replica() anymore for backfill operations since a backfill isn't
a replica due to acting set change.
Now always start_flush at Stray in addition to ReplicaActive/GetLog
Track multiple flushes with flushes_in_progress count
Add all flush event handling in new on_flush_received()
Remove unused "flushed" event local variable
Temporary additional messages for testing