setContext link race condition can leak subscriptions

[sofianhn]

Intended outcome:
Subscription cancelled before operation.setContext functions are resolved as part of the setContext link don't advance to the next link creating phantom subscriptions.

Actual outcome:
A Race condition can lead to phantom subscriptions created which results in memory leaks.

How to reproduce the issue:
The test in the PR attached to this ticket is a repro case.

Versions

System:
OS: macOS 10.15.7
Binaries:
Node: 14.8.0 - /usr/local/bin/node
Yarn: 1.22.4 - /usr/local/bin/yarn
npm: 6.14.7 - /usr/local/bin/npm
Browsers:
Safari: 14.0