Updates for new dstore api (in progress)

The new dstore API (in master) is incompatible with v0.1.0, and vice-versa.   These changes are to make are code with the master branch of dstore.   They should be checked in on the next release of dstore.

Even with the changes, I'm still getting test failures, but it's a start.

I deliberated whether the `map()` call should be in `fetch()`, or before `fetch()`.   Probably it should be before.  I think a `sort()` operation might actually make sense after the `map()` call rather than before it, because (for a List etc.), the user will expect to sort on the visible computed columns, rather than sorting on what's originally in the store.

Also, I think `_itemToRenderItem()` calls in `_itemUpdated()` and `_itemAdded()` could be skipped if we started tracking (the `track()` call) after the original `map()` call.   Probably doesn't matter much but maybe that's a little cleaner.