Investigate possibility of multithreaded execution as opt-in vs. opt-out for NET 10

[JanProvaznik]

trafficstars

Issue for conversation and documenting design decisions made during multithreaded epic that might be problematic. And keeping track of the cost/benefit.

Ideally we'd have single-process execution as opt-out to bring the performance benefit seamlessly to as many customers as possible. However to do that it has to be highly compatible to avoid frustration and enable fearless upgrades.