Fix throttle not clearing its internal interval when unsubscribed

[Avaq]

When a throttled stream is unsubscribed, its internal throttle interval is not cleared, and the reference to its ID is removed, meaning it's also not cleared at the end of the timeout.

This is essentially a memory leak, where the interval will live on in the event loop forever without being cleared.

This PR adds a call to ThrottleOperator#clearInterval() in its _stop method, clearing the interval and freeing it from the event loop.

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I checked if the same problem exists for debounce, but it looks like it was already fixed in #296:

xstream/src/extra/debounce.ts

Lines 18 to 22 in fee5f0d

	_stop(): void {
	this.ins._remove(this);
	this.out = null as any;
	this.clearInterval();
	}