[AGENT++] How to check whether an MibTable::update is needed or not

[qinyd at tom.com]

Hi Frank and all list members:
Since the method MibTable::Update may be called once for each subrequest of the given request.
How to check whether an MibTable::update is needed or not

Some examples wrote below,but it seem wrong.

  if (currentRequest == req->get_request_id()) return;
  currentRequest = req->get_request_id();