datastore: GC shouldn't have to return dropped RowIds

[teh-cmc]

The GC currently returns all the RowIds that were purged so that secondary datastructures in the viewer can be purged in turn.

But these secondary datastructures shouldn't be indexed by RowId to begin with but by time instead, making it possible to purge them using cut off times (like we do for e.g. the timeline widget).

[teh-cmc]

Cutoff times are Impossible now that the GC is driven by RowId order (clients' wall-clocks), which is the only solution we've found to fairly distribute GCs across timelines.

Individual timestamps are possible, and is in fact what we're already doing in most cases.