Follow-up on various issues such as #38942 and in the context of https://servo.zulipchat.com/#narrow/channel/263398-general/topic/Fixing.20indexeddb.20intermittency.20.28architecture.29/with/556682047

Our current implementation does not implement the spec from a general architectural perspective. Many algorithms that the spec runs "in-parallel" are run on the script event-loop instead.

Before #40648 this was a source of intermittency in the WPT test results, but the solution still leaves the architecture lacking, in particular blocking the script-thread(event-loop in spec parlance) means the UI and any other JS computation would freeze while a transaction is being processed.

Besides this, although I haven't identified the specifics, it also likely means there are some tests that could never pass unless the architecture is fixed.

This is a meta issue where I'll add items(perhaps with links to other issues) as I discover them.