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 remote-tracking branch 'origin/wip-9625' into wip-log-crash-firefly-wip-9574