fixes stack overflow possibility with merge operators

[geoffmacd]

Fixes https://github.com/ReactiveX/RxSwift/issues/2615 See issue for details

when completing Merge operator iterations (concat(), concatMap() and merge(maxConcurrent:), fixes subscribing immediately to the next in the queue, which can produce values immediately which can re-enter and cause stack overflows. ultimately uses CurrentThreadScheduler and the isScheduleRequired prop to know if it needs to schedule or not. This is similar to Producer.

All tests pass