importer: fix writeback of transactions during which readCurrent() was used

Contrary to FileStorage, NEO remembers uses of readCurrent().

neo/storage/database/importer.py

@@ -813,7 +813,10 @@ class TransactionRecord(BaseStorage.TransactionRecord):
     def __iter__(self):
         tid = self.tid
         for oid in self._oid_list:
-            _, compression, _, data, data_tid = self._db.fetchObject(oid, tid)
+            r = self._db.fetchObject(oid, tid)
+            if r is None: # checkCurrentSerialInTransaction
+                continue
+            _, compression, _, data, data_tid = r
             if data is not None:
                 data = compress.decompress_list[compression](data)
             yield BaseStorage.DataRecord(oid, tid, data, data_tid)

neo/tests/threaded/testImporter.py

@@ -238,6 +238,7 @@ class ImporterTests(NEOThreadedTest):
             # New writes after the switch to NEO.
             last_import = -1
             for i, r in enumerate(after(r)):
+                c.readCurrent(r)
                 t.commit()
                 if cluster.storage.dm._import:
                     last_import = i