ot::DataPollSender::SendFastPolls() method

Asks data poll sender to send the next given number of polls at a faster rate (poll period defined by `kFastPollPeriod`). This is used by OpenThread stack when it expects a response from the parent/sender. If @p aNumFastPolls is zero the default value specified by `kDefaultFastPolls` is used instead. The number of fast polls is clipped by maximum value specified by `kMaxFastPolls`. Note that per `SendFastPolls()` would increase the internal reference count until up to the allowed maximum value. If there are retransmission mechanism in the caller component, it should be responsible to call `StopFastPolls()` the same times as `SendFastPolls()` it triggered to decrease the reference count properly, guaranteeing to exit fast poll mode gracefully. Otherwise, fast poll would continue until all data polls are sent out.