Apply hang protection to HashSet<T> that we added to Dictionary<K,V>

[danmoseley]

HashSet is likely prone to the same expensive hang as it uses essentially the same datastructures.

We should port dotnet/coreclr#16991 to Hashset.

cc @benaadams

[danmoseley]

https://github.com/dotnet/corefx/issues/28123

[danmoseley]

I see @vancem pointed out it has been reported to have been hit in cloud services using HashSet also. We should certainly port, then.

@safern perhaps you'd like to do this?

[safern]

@safern perhaps you'd like to do this?

Porting it now.

[mjsabby]

Can we also port this to 2.0?

[danmoseley]

This is not going to meet the bar to port to 2.0 given that it will be very soon superseded by 2.1 and our support policy