R1:50ed42761ce9

Don't issue a bazillion queries to load Differential object lists

Summary:
Ref T3496. Currently, we call loadAssets() on each revision table, which invokes a new revision query and a pile of subqueries.

Instead, add `needFlags()` and `needDrafts()` to `RevisionQuery`. Some day these could perhaps be more generic.

Test Plan:
- Viewed home, differential, etc., no longer saw 9203809238 queries being run for no reason.
- Drafts and flags still appear properly.

Reviewers: btrahan

Reviewed By: btrahan…