Linearize streaming client to prevent flaky tests

[erickpintor]

There's one class of flaky tests which emerges from the fact that a read-only query to create a stream token will contain a snapshot time instead of a transaction time as its starting time. Subsequent writes may come back with a txn time lower than the stream's starting time, which can't be used as a resumable point. Making the test client linearized just makes testing more reliable here.

[erickpintor]

We ended up adding fix on the database side that is meant to prevent this scenario. I'll follow up on the fix there.