fixes stack overflow possibility with merge operators

[geoffmacd]

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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

[danielt1263]

This looks like a good candidate for a dot release @freak4pc. What do you think?

[freak4pc]

Hey @geoffmacd, sorry for the long delay on this.

1. Did you use your own fix in prod by now? Is it stable?
2. Is the code ready from your perspective ?
3. Let's rebase / merge with main and have CI run all the tests to make sure we're good.

Thanks!

[geoffmacd]

Hey @freak4pc

Sorry for delay.

1. Yes this has been integrated in dropbox for 6 months which heavily uses Rx and is stable.
2. yes.
3. Will do.

[freak4pc]

Much appreciated, thank you :)

[geoffmacd]

ok, updated from main. ready to merge