PG: make the reservation sets more descriptively named

These sets won't precisely be the backfill_targets or actingbackfill
shortly.

%s/sorted_backfill_set/remote_shards_to_reserve_backfill/g
%s/acting_osd_it/remote_recovery_reservation_it/g
%s/sorted_actingbackfill_set/remote_shards_to_reserve_recovery/g

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: recover from each osd at most once

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2349 from athanatos/wip-9240

Wip 9240

Reviewed-by: Sage Weil <sage@redhat.com>