Refactor parallel operation manager

[nohwnd]

Description

There is still a tiny chance that we will attempt to schedule some work twice, but this was never observed, so I am not making it part of #4629 .

In ParallelOperationManager we should figure out work to do under lock, set a property on the slot that we are processing it, and then leave the lock. Under the lock we should not read anything from the slots, just loop over the collections of work we got.

This way we avoid reading ShouldPreStart (or similar property) from 2 threads just at the wrong moment, where both threads will try to start the work.

[nohwnd]

fyi @cvpoienaru this should probably be done before re-enabling the pre-start. It is not a lot of work, just a small reshuffle.