crimson: retain reference to WaitForObjectRecovery until blocker resolves

crimson: retain reference to WaitForObjectRecovery until blocker resolves

PGRecovery::on_global_recover destroys the map entry without waiting for
the future returned from

    seastar::future<> wait_for_recovered(BlockingEvent::TriggerI& trigger) {

This commit changes WaitForObjectRecovery to be refcounted and retains a
reference until the future resolves.

Fixes: https://tracker.ceph.com/issues/55565
Signed-off-by: Samuel Just <sjust@redhat.com>