qa: document a rare random failure in testExport

Traceback (most recent call last):
  File "neo/tests/functional/testClient.py", line 241, in testExport
    self.assertEqual(dump, self.__dump(neo_db.storage, list))
  File "neo/tests/functional/testClient.py", line 210, in __dump
    for t in storage.iterator()}
  File "neo/tests/functional/testClient.py", line 207, in <dictcomp>
    return {u64(t.tid): sorted((u64(o.oid), o.data_txn and u64(o.data_txn),
  File "neo/client/iterator.py", line 69, in iterator
    max_tid, chunk = app.transactionLog(start, stop, CHUNK_LENGTH)
  File "neo/client/app.py", line 841, in transactionLog
    Packets.AskTIDsFrom(start, stop, limit, offset))
  File "neo/client/app.py", line 296, in _askStorageForRead
    return askStorage(conn, packet)
  File "neo/client/app.py", line 164, in _askStorage
    return self._ask(conn, packet, handler=self.storage_handler, **kw)
  File "neo/lib/threaded_app.py", line 144, in _ask
    _handlePacket(qconn, qpacket, kw, handler)
  File "neo/lib/threaded_app.py", line 133, in _handlePacket
    handler.dispatch(conn, packet, kw)
  File "neo/lib/handler.py", line 72, in dispatch
    method(conn, *args, **kw)
  File "neo/lib/handler.py", line 208, in error
    getattr(self, Errors[code])(conn, message)
  File "neo/lib/handler.py", line 227, in backendNotImplemented
    raise NotImplementedError(message)
NotImplementedError: neo.storage.database.importer.ImporterDatabaseManager does not implement getReplicationTIDList

neo/tests/functional/testClient.py

@@ -238,6 +238,9 @@ class ClientTests(NEOFunctionalTest):
         self.neo.start()
         neo_db, neo_conn = self.neo.getZODBConnection()
         self.__checkTree(neo_conn.root()['trees'])
+        # BUG: The following check is sometimes done whereas the import is not
+        #      finished, resulting in a failure because getReplicationTIDList
+        #      is not implemented by the Importer backend.
         self.assertEqual(dump, self.__dump(neo_db.storage, list))
 
     def testIPv6Client(self):