fix: don't assert on unordered publishes after publish error

[pradn]

Unordered batches that threw errors on invoking the publish RPC led to subsequent assertion failures in thread.py::publish(). These assertion failures were legitimate. Publish calls should not be called on batches that have already been committed. (In this case, they were committed and then returned an error.)

This pull request fixes the underlying issue by clearing out the UnorderedSequencer's _current_batch on commit(). This ensures that no subsequent publish() calls will be called on that batch since the reference to it is lost.

I add a new "test_publish_after_batch_error" test for this scenario.

The OrderedSequencer does not have this issue. On error, it cancels all batches and pauses the ordering key. The existing "test_batch_done_unsuccessfully" test captures this scenario.

Fixes #48

[plamut]

(reviewing right now, will merge myself if all is fine and the CI is done by then)

[pradn]

(reviewing right now, will merge myself if all is fine and the CI is done by then)

Thanks Peter!

[plamut]

The fix works and the code looks good.

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