PG: release backfill reservations if a backfill peer rejects

Also, the full peer will wait until the rejection from the primary
to do a state transition.

Fixes: #9626
Backport: giant, firefly, dumpling
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2658 from athanatos/wip-9625

PG: release backfill reservations if a backfill peer rejects

Reviewed-by: Sage Weil <sage@redhat.com>