trim: gc and client barrier for reclaim

[dotnwat]

there is an issue with trim where we leave behind old objects as placeholders. just the object and the header, but no content. these can be cleaned up after some sufficiently long period that clients don't hold state referring to them. however... we could use something like rados watch/notify to add a barrier that updates inflight clients to allow objects to be reclaimed. if we do this, we should be sure to use a backend interface that doesn't depend on watch/notify-type implementation details specifically.