Provide a callback (event) API for implicit yield of concurrent frame

[ggleyzer]

In Ecstasy language, the Concurrent mixin is used to annotate classes, properties, and methods, to indicate that the
class, property, or method is safe for concurrent/reentrant execution. An implementer of a Concurrent method needs to be mindful of potentially yielding calls made within the concurrent section of the code and not rely on potentially invalidated local state. @markandrewfalco suggested an extension of Service API that would provide a service callback only if that potential yield has realized and some of the previously collected state might have become stale, thus allowing the service implementer to perform necessary book-keeping only when necessary.