os_unfair_lock_lock is being called from AppDelegate

[fampaySiddharth]

If I use debug google service plist for Firebase and initialize with it. Then Merge throws a os_unfair_lock_lock error, sometimes reactor.dispatch also throws this

[fampaySiddharth]

The 'retainSink' is causing the issue.

public func uploadFCMToken() {
    _ = myRepository().uploadFCMToken().retainSink() <- This retain sink
  }

[vmanot]

This is related to the Merge framework (and also should be fixed with the recent refactor of PassthroughTask).