Garbage collect old unused index segments

[loganmhb]

As it stands, old index segments sit around taking up space without being used for anything. It should be possible to track whether a segment is in use or not (and perhaps at what time it was last in use) and delete any segments that are older than a particular time.

In Datomic, this is a manually-triggered operation, because it's difficult to know whether some long-running client process might be hanging on to an old database value that still needs a particular segment. The user is in a better position to know what timescales are safe than the database itself.