fix(pubsublite): fix committer races

[tmdiep]

Races:

1. When Stop is called before Terminate when there are outstanding acks, the committer did not transition to serviceTerminated. Terminate clears outstanding acks, so unsafeCheckDone needs to be called to check whether the committer can now shut down.
2. If there is a pending commit, and Stop or Terminate are called just before the stream is connected, it can send a commit to the server before onStreamStatusChange performs the necessary state initialization for the streamConnected signal. So we add an enableCommits flag to ensure onStreamStatusChange processes the streamConnected signal first.

Simplified TestAssigningSubscriberIgnoreOutstandingAcks, which detected these races.

Fixes #3791.