Consider removing VirtualField.computeIfNull()

[trask]

Carrying PR discussion forward from #4262 (comment).

The atomicity guarantee for VirtualField.computeIfNull requires a lock, which has led to problems (#4188).

Just dropping the atomicity guarantee is an option, but it could be confusing to have a method that looks like it provides atomicity signature-wise, but really doesn't (#4192 (comment)).

And if it doesn't provide atomicity anyways, usage of computeIfNull can be replaced by calls to get and set.

[mateuszrzeszutek]

I've started working on that and removed almost all usages of computeIfNull - the only one left is ExecutorAdviceHelper#attachContextToTask() - I think it may actually be needed here; it ensures (atomically) that a PropagatedContext is always present, and the PropagatedContext class actually manages the state/context.

I think we can remove the PropagatedContext class together with the computeIfNull method if we simply provide a compareAndSet method on the VirtualField (using an AtomicReferenceFieldUpdater in the generated implementation)

[mateuszrzeszutek]

The compareAndSet method is another reason for implementing #4241 - since implementing it with a Cache would be quite difficult.