Fix resharding cleanup datarace with update queue

[timvisee]

Two systems conflict:

1. resharding propagate deletes stage - deletes points that don't belong in (old) shard anymore
2. update queue - operations pending in update queue that did not land in segments yet

During resharding up, we copy a portion of points from all existing shards into a new shard. We leave them in place for some time to prevent read consistency problems. At the end of resharding we delete these moved points as they don't belong in the old shard anymore.

Together with the update queue system this allows a data race. Queued operations are not cleared. But, these queued operations may still create/update points that don't belong in the shard anymore. So, after clearing old points is finished the same old points may land in the shard again from the update queue.

This PR adds a test to assert this behavior. It fixes the problem by making every clear operation wait for the current update queue to be applied with sort of a plunger operation. The test fails before the fix, and succeeds with the fix.

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