Set loading to false if data is already in cache

[vishwam]

This is a work-in-progress PR to fix #5869 (and possibly #5835 and #5947): useQuery currently returns loading: true even if the data is already in the cache (and then re-renders with loading: false, causing a flash with the loading UI.)

I narrowed it down to ObservableQuery.getCurrentResult - it gets the data and partial from the QueryManager (which gets it from the cache). If we know we already have the complete data, I think we can short-circuit the queryStore.get() flow and return result with loading: false. I've tested this locally with various scenarios, and am able to get back to v2's behavior (only one render with loading: false).

Please let me know if this is the correct approach. If it is, I can fix the tests and send an update to this PR (it's failing because currentResult no longer matches observableQuery.next - I'd also like some pointers on how best to fix this.)

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[benjamn]

At the very least this logic needs to take the fetchPolicy into account. I can only imagine this being appropriate for the cache-first fetch policy, where the presence of complete cache data means there won't be a network request. For policies like cache-and-network, getting data from the cache definitely does not mean the request is finished!

As a side note, this change triggered a lot of very much related test failures. If you're going to make a small change that has big consequences, at the very least you need to spend some time really understanding why those tests are failing. Just making the most obvious changes to get them to pass is not going to work for us.