Refacto RxJS-based ABRManager into a SharedReference-based AdaptiveRepresentationSelector

[peaBerberian]

After #962 and #1042, this is another huge pull request which tries to progressively reduce our dependency on the RxJS library to improve code "approachability". You can look at the two previous (since-merged) PR for reasons leading to this.

Here RxJS and associated concepts have been completely removed from the src/core/abr directory, which concerns our Adaptive BitRate-linked code.
On that subject, in the same vein than the eme directory has been renamed decrypt and the EMEManager the ContentDecryptor - so its more explicit and clear to newcomers and external contributors - the abr directory has been renamed adaptive and the ABRManager, the AdaptiveRepresentationSelector.

The key points here are:

• Because the adaptive code use many event listeners, a lot of code in compat/event_listeners.ts has been updated so it returns IReadOnlySharedReference s instead of RxJS Observables.
  Shared references can be seen as [very] simplified BehaviorSubjects when compared to RxJS. Their read-only flavor cannot update the value, they can only get the current value or "listen" (subscribe) to value changes (because the event listener should not have the need to emit them).

  Simple shared reference description for those not that familiar with BehaviorSubjects: SharedReferences are JS objects with a value inside them (any value: a number, boolean, object, another shared reference etc.), that can be updated and read at any time (by anyone in possession of this object), with also the possibility to register a callback so it is called each time its stored value is updated.

• adaptive estimates are also returned as an IReadOnlySharedReference - so the AdaptationStream can easily create a RepresentationStream each time the Representation estimate changes through a callback.

• A third argument has been added to our EventEmitter's addEventListener method: an optional CancellationSignal.
  This allows to easily remove the listener when the signal emits - CancellationSignal being the preferred solutions for now for cancellation handling in the player (previously happening implicitly through Observable unsubscription).
  Because we do not want to leak that supplementary argument to the API/TypeScript types, the public API (core/api/public_api.ts) redefines its own addEventListener method on top so it can explicitly remove the third argument.