Fixed ReplaySubject leak

[akarnokd]

Fix for issue #939.

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[mttkay]

I can confirm this fixes #939 for me, thanks @akarnokd !

This might in fact be the final nail in the coffin for any workarounds we used on Android to prevent Activities from leaking, this is great.

[mttkay]

@benjchristensen do you think we could get this in for 0.17.1, that would be fantastic. It would simplify reasoning about requirements for Android operators a lot, since the use of cache and replay is quite common on Android and some design decisions for OperatorObserveFromAndroidComponent were made purely due to the existence of this bug (no one actually thought to question the correctness of the underlying operator back then, hah.)