Once SERVER-119607 is done, we can implement background cleanup of the durable chunk entries.

This will help cleanup the durable state of chunks that are no longer reachable based on the current oldest WT timestamp possible. That is, if a chunk is only visible on the current shard up to timestamp T=10 and the current oldest WT timestamp allowed for PIT reads is T=20 then we don't need to store the chunk metadata durably and can be deleted from disk.

Updating the in-memory state is potentially out of scope for this ticket since not doing anything there is a correct solution since it will go away on the next full invalidation/restart.