osd: 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
Use is_acting() in proc_replica_info() because this is before actingbackfill set
Remove backfill_targets from 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.

fixes: #5855

Signed-off-by: David Zafman <david.zafman@inktank.com>