Lock Free Pinning and (possible) GC.

We should try and move towards lock free pinning and possible GC.

I believe that this is possible but will require some very careful thought. (In my original filestore implementation which never got merged I implemented something very similar to lock free gc in some of the utilities.) It is likely also to require some additional functionally from the block/data store.

My ideas for implementing this require the ability to take snapshots of the datastore. There many be other ways.