refactor: Reduce memory consumption and simplify inner and outer subscription

[benlesh]

• Reduces memory pressure by no longer capturing outer values where they are not needed. For example, every inner subscription in mergeMap was retaining the outer value, if the outer value was a large array, that could add up
• Makes subscribeToResult less ridiculous, I am not sure what I was thinking there 🤦
• Leaves a few operators with more complex inner subscriptions, to be refactored later.
• Removes some redundant code that removed inner subscriptions manually, this was necessary in the early days of RxJS 5, but has not been necessary in a long time

Explainer

So the old OuterSubscription is now called ComplexOuterSubscription, and InnerSubscription is ComplexInnerSubscription.

Basically, back when we had all sorts of additional "resultSelectors" (pre v6), those result selectors relied on data that we captured with the InnerSubscription. After removing resultSelectors, that was no longer needed in the majority of cases, BUT it was still there to keep this "one inner subscription process to rule them all" sort of thing we had going.

Unfortunately, those trapped outer values can be costly. Imagine this scenario:

import { of, timer } from 'rxjs';
import { mergeMap } from 'rxjs';

const data = new Array(1000000).fill('test');

of(data).pipe(
  mergeMap(d => timer(1000000))
)
.subscribe(console.log);

In the above scenario, the subscription to of(data), as a source begins and ends synchronously. BUT!!! even though we don't even use that huge array in the inner observable we return in the mergeMap, that array is retained in the InnerSubscriber's outerValue property, which is created by the merge map... and held, in this case, for 1,000 seconds! Not great.

This PR solves that by no longer retaining the outerValue unless necessary.

There's a long tail of one-offs that we'll need to address. But they are commented on the ComplexOuterSubscriber implementation and its methods.

[cartant]

Some comments, opinions and nits.

[niklas-wortmann]

LGTM