Remove RxJS from PlaybackObserver to fix potential leak

[peaBerberian]

While testing the future v4.0.0, I noticed a memory leak which way well also affect previous RxPlayer versions.

The leak was that the PlaybackObserver may be still emitting even after playback has been stopped. The actual impacts are non-removed DOM event listeners, unnecessary logic running every seconds or so, and noisy logs

After a lot of console.log-based investigation (as RxJS make the call stack inexploitable as well as step-by-step breakpoints), it seems that the issue is either due to a RxJS bug or of a poor comprehension I have of the shareReplay operator (I would go for the former, but I'm biased!).

Basically, the leaking Observable (the one that is still running despite not being needed anymore) did not have any subscriber left. It had a shareReplay associated to it, with a bufferSize attribute set to 1 and much more importantly a refCount attribute set to true.

The way I understand the refCount option is that the Observable will be shared (it is a shareReplay after all) until no subscriber are left (the reference counter drops to 0) at which point the Observable will be unsubscribed from. This is also what I understand from the code documentation here:
https://github.com/ReactiveX/rxjs/blob/47fa8d555754b18887baf15e22eb3dd91bf8bfea/src/internal/operators/shareReplay.ts#L41-L43

Yet it didn't seem to be the case. In my situation, no subscriber seemed to be left yet the Observable was still performing its side-effects, such as event listening and logs.
As no subscriber was left, nothing else happened, so the damage was limited.
Moreover, removing the shareReplay operator seemed to properly unsubscribe the Observable on which it was applied, which may be another proof that no subscriber was left (though it may not be by itself, but I can assure you that it was the case).

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Because it is complex to set-up reproduction steps, because I already found an open issue on the shareReplay operator and because we want to stop using RxJS anyway altogether in the long term, I preferred spending my time today rewriting the PlaybackObserver so it does not depend on a ReplaySubject and even on RxJS.

This was much easier than what I would have thought.
At least two subtle differences exists in behavior with the previous PlaybackObserver though:

1. Previously the PlaybackObserver started emitting its first event on the first subscription, and (theoretically) restarted emitting if re-subscribed after being totally unsubscribed from anyone.

   Now, the PlaybackObserver is started right away on instanciation.

2. Previously, the PlaybackObserver (theoretically!!!!!) stopped emitting when all subscribers unsubscribed from it.

   Now, there is an explicit stop method, called by the public_api.ts file (which is the file creating the PlaybackObserver in the first place, so it makes sense).