4.x: Make TailRecursiveSink lock-free and have less allocations

[akarnokd]

This PR changes TailRecursiveSink to a lock-free algorithm as well as reduces the allocations in the class by inlining various IDisposable fields.

The aim of the class is to make sure OnComplete or OnError in Concat, Catch and various other operators don't trigger an infinite recursion but stay on the level.

This could have been solved with a straightforward trampoline like this Concat implementation, however, there is a small complication. The recursion may be not only on the termination side, but on the existence of the inner IObservables.

For example, to avoid yet another level of recursion due to graphs like Concat(Concat(a, b), Concat(c, d)), the Extract method can get another level of IEnumerable<IObservable<TSource>> on the current level, and the concatenation should resume on the inner Concat.

The Stack is there to allow returning to an outer enumeration and resume it. I don't know what the length stack was supposed to do; the algorithm doesn't have to know how long each IEnumerator is. Was it some kind of optimization for Lists and arrays avoiding MoveNext() == false and thus IEnumerator.Dispose()? The former should be as costly as an index comparison and the latter should be no-op anyway.