qa: rewrite testReplicationBlockedByUnfinished as a threaded test
It is extended to check that the storage is only notified about the transactions that existed at the time it asked for them. Otherwise, Replicator.transactionFinished would be called more than once, and `self.ttid_set.remove(ttid)` would raise KeyError. The functional version also contained an annoying 'sleep(10)'.
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