Fix cleanup observer blocking unsubscribe (#6985)

[javier-garcia-meteologica]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

Fixes #6985

Cleanup observers were blocking removeObserver from executing this.sub.unsubscribe(). Therefore some queries were left hanging until completion. This patch puts the observers that should not block unsubscription into a separate set, shadowObservers.

[apollo-cla]

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[dylanwulf]

May also fix #4955

[benjamn]

@javier-garcia-meteologica Thanks for fixing this!

I'm going to go ahead and merge this PR into release-3.3 and publish another beta (so you can try these changes out), but please do let me know if you disagree with any of the changes I made.

[javier-garcia-meteologica]

Hi @benjamn

I've tested the changes and unfortunately queries are not being aborted. I've open a PR to fix it in #7170.

The problem is that there are still 2 observers when removeObserver is called: the query observer and the cleanup observer.

  public removeObserver(observer: Observer<T>, quietly?: boolean) {
    if (this.observers.delete(observer) &&
        this.observers.size < 1) {
      if (quietly) return;
      if (this.sub) {
        this.sub.unsubscribe();

This is easy to fix, just decrement --this.addCount and check this.addCount < 1.