Refactor query latch

[zetanumbers]

Removed unnecessary mutex for cycle errors.

[rustbot]

r? @madsmtm

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[zetanumbers]

r? @nnethercote

[Zalathar]

Could you explain what this change is doing, conceptually?

Right now there’s no way to know what’s going on, other than trying to reverse engineer the textual changes.

[nnethercote]

Could you explain what this change is doing, conceptually?

I was going to ask the same thing. A good rule of thumb is "make life as easy as possible for your reviewer". A bit of context and explanation about the PR helps a lot.