storage.handler.client: Create a transaction when checking current serial.

This fixes a bug causing a crash during tpc_finish phase when a storage
involved in a transaction does not receive any object, but receives at
least one CheckCurrentSerial request: no transaction was registered, and
storage would fail to lock transaction when requested by master during
tpc_finish phase.

neo/storage/handlers/client.py

@@ -184,6 +184,7 @@ class ClientOperationHandler(EventHandler):
         conn.answer(p)
 
     def askCheckCurrentSerial(self, conn, ttid, serial, oid):
+        self.app.tm.register(conn.getUUID(), ttid)
         self._askCheckCurrentSerial(conn, ttid, serial, oid, time.time())
 
     def _askCheckCurrentSerial(self, conn, ttid, serial, oid, request_time):