common/sharedptr_registry.hpp: add remove

remove() can be used to clear an entry before all of its
references are removed.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: remove the deleting state ref on cancelation

When the PG deletion is canceled, remove the DeletingState ref
from the registry.  Subsequent operations do not need to wait for
remaining references to go away.

Signed-off-by: Samuel Just <sam.just@inktank.com>
no need to preserve DeletingStateRef after _process

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: no need to wait on DeletingStateRef for flush

Signed-off-by: Samuel Just <sam.just@inktank.com>
reverse order