storage: fix history() not waiting oid to be unlocked

This fixes a random failure in testClientReconnection:

Traceback (most recent call last):
  File "neo/tests/threaded/test.py", line 754, in testClientReconnection
    self.assertTrue(cluster.client.history(x1._p_oid))
failureException: None is not true

neo/storage/handlers/client.py

@@ -172,8 +172,11 @@ class ClientOperationHandler(EventHandler):
     def askObjectHistory(self, conn, oid, first, last):
         if first >= last:
             raise ProtocolError('invalid offsets')
-
         app = self.app
+        if app.tm.loadLocked(oid):
+            # Delay the response.
+            app.queueEvent(self.askObjectHistory, conn, (oid, first, last))
+            return
         history_list = app.dm.getObjectHistory(oid, first, last - first)
         if history_list is None:
             p = Errors.OidNotFound(dump(oid))