Fix some polling/StrictMode issues, along with some general refactorings

[brainkim]

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

Notes

Continuation of #8346

Spent some time deleting Reobserver, as well as fixing some polling misbehavior.

Most of the test changes are related to fixing StrictMode stuff. The timings of moving ObservableQuery logic to useEffect made it so that we need little bits of delay to get the tests working. The behavior doesn’t seem to have changed in any meaningful way, we just skip some renders when we refetch or rerender too soon. There was one test which just didn’t make sense, was never failable in any meaningful sense, and used timer mocks in a way which messed with other tests so I deleted it. Forgive me.

This should probably make Apollo Client “strict mode” safe, though it is roughly impossible to tell for certain.

Fixes

• useQuery pollInterval props still triggered even after unmount #6391
• stopPolling doesn't work when using React StrictMode #7221
• useQuery pollInterval props still triggered even after unmount #6391

TODO: Investigate if this fixes

• Apollo Client 3 - optimistic response with pollInterval doesnt work. #6858
• Reactive variables stop working with Fast Refresh #6661
• Fast-refresh issue #5870
• useLazyQuery polling? #5140

[benjamn]

Thanks to the granularity of your commits, @brainkim, these changes were actually pretty easy to review! I left a few comments in-line, but nothing merge-blocking.

Speaking of merging, feel free to rebase and squash commits together before merging if you like, but I think I'd prefer to keep these separate commits (with a merge commit), instead of squashing the whole PR.

[hwillson]

This looks great @brainkim - thanks very much!

(just a gentle reminder to update the CHANGELOG 👍)

[benjamn]

@brainkim I just released this in @apollo/client@3.4.0-rc.13, fyi.