fix(sync-v2): Stop streaming if current block becomes voided

[msbrogli]

Motivation

Fix one of the conditions that makes the tests/simulation/test_simulator.py::SyncV2RandomSimulatorTestCase::test_many_miners_since_beginning flaky.

If we do not stop the streaming, the later call to get_next_block_best_chain() will raise an assertion error since the next block will be voided.

Acceptance Criteria

1. Stop streaming blocks if the current block gets voided.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[jansegre]

Does this really improve it? The test matrix failed in all but one job (at least on the first run). Also I think we've discussed this before, we previously had this behavior (stopping a stream when it reaches a voided block) but we changed it, and I don't remember why.

[glevco]

@jansegre maybe it helped with one test, but others failed in the run. One way to check is this adding, for example, @flaky(min_passes=20, max_runs=20) to force the supposedly fixed test to run 20 times and pass all of them. We can run this locally just to check if all runs are passing (but keep it out of the PR).