fix(event cache): wait for the initial previous-batch token, when storage's enabled

[bnjbvr]

If during back-pagination, we lazy-load a chunk from storage and realize there's no previous chunk, we might think we've reached the start of the timeline and that we're done. This is not true, if this was the first default chunk, and we never waited long enough to get the initial gap from sync. This patch fixes that, and includes a regression test showing the error.

Part of #3280.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.13%. Comparing base (54ab46d) to head (9c1e119).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4724      +/-   ##
==========================================
- Coverage   86.13%   86.13%   -0.01%     
==========================================
  Files         291      291              
  Lines       34300    34304       +4     
==========================================
+ Hits        29546    29549       +3     
- Misses       4754     4755       +1     

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[Hywan]

Thanks for the test. I don't understand one thing but it's documented and tested, I approve this PR.

[Hywan]

Why is it here? I feel like this is not the correct place, but it's a gut feeling.

[bnjbvr]

This is a central location where to do it, since we'd need to do it every time after the linked chunk has been modified / after a gap has been potentially added. That happens during sync and when resolving a gap, so… here's likely the right place.

I'm tempting to rethink the waiting for a prev-batch token mechanism, because 1. it causes lot of code everywhere, 2. it's likely used not very often.

[Hywan]

Thanks for the explanation. Makes sense to me.