fix(window): don't track internal window subjects as subscriptions.

[trxcllnt]

Description:
This PR removes behavior in the window operators that tracked the window Subjects as Subscriptions. This could lead to race conditions where the Subscriber unsubscribes while the window operator is still active and attempting to send messages to the windows.

Related issue (if exists):
#1698

[benlesh]

I don't understand why this test went away. Can you elaborate? Thanks.

[trxcllnt]

@Blesh now that window's internal Subjects aren't being tracked as if they were Subscriptions, their unsubscribe method is never called when the subscriber disposes. This test attempts to subscribe to one of the window Subjects after the outer window subscription has been disposed, assuming the WindowSubscriber has called unsubscribe on the Subject. This PR invalidates that assumption.

[benlesh]

Makes sense, thanks

[benlesh]

It seems like #1705 resolves #1698. @trxcllnt, can you explain the relationship between #1698 and this PR?

[trxcllnt]

@Blesh #1705 resolves #1669 and partially fixed #1698, after which I discovered an unrelated race condition. The window operators would continue to next or complete the internal window Subjects, even if a downstream subscriber unsubscribed in response to a next or complete event. Since the internal Subjects would be disposed at the time the downstream subscriber unsubscribed, attempting to message them again would throw ObjectUnsubscribedErrors. Minimal repro example:

Observable.range(0, 2)
  .windowCount(1).mergeAll()
  .take(1).subscribe();

[benlesh]

Looks good to me, @trxcllnt.

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