ChangeData: Allow async batching in ensureAllPatchSetsLoaded

Borrow the trick from ChangeJson.loadReviewed() where we can fire off
queries into ResultSets asynchronously (if supported by the backend)
and then memoize them at the end of the batch.

Change-Id: I9ddbbfb6d8d59f5be64d852fe2d9e2c74143e1ae