chore: stop using piscina worker methods for runEventPipeline

[tiina303]

Problem

We don't really need to use piscina here and it's creating unneccesary confusion reading the code

Changes

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

How did you test this code?

[bretthoerner]

Nice, makes a lot of things easier to follow.

[bretthoerner]

It's confusing that we store it in the constructor and also here. Does just 1 or the other not work?

[tiina303]

right, forgot to remove it from here ... I added if before creating a function outside of the class that call locations can use

[bretthoerner]

Rebased this on master so I could try it out on my PR.

For historical records: @xvello and I noticed in #17932 that the Piscana layer was doing funny things with our objects. What should have been an array of Promise objects was turned into an array of plain Objects... so either the promises were dropped or they were awaited, either one is scary.

We should merge #17932 (which includes the commit from this PR) on Monday so we don't risk a behavior/performance change on Friday.

[xvello]

Thanks for the investation Brett. @tiina303 please don't merge this PR by itself, we'll merge it as part of #17932 to avoid a regression: we'd fail to process messages that are too big for the destination topic.