Reduce latency by reducing on-device buffers

[wmanley]

The DTU-215 seems to refuse to transition from state HOLD to state SEND (using SetTxControl) unless there are 2MB of data in its FIFO and the total size of the FIFO seems to be about 16MB. This is too big as it means we need to pre-roll at least 2s worth of a DVB multiplex before we can enter the PLAYING state and once the buffers are full and DtOutpChannel::Write starts blocking we may have introduced >16s worth of latency.