Bidirectional PAD communication protocol

The current ODR-PadEnc to ODR-AudioEnc communication is done through a nonblocking FIFO.

The proposal is to replace it by a UDP-based request/reply protocol.

For every audio frame, the ODR-AudioEnc checks if a block of PAD data is available by doing a nonblocking recv() on the UDP socket. If yes, it inserts it into the audio frame; if not, the audio frame will not contain PAD. In any case, it sends a request to ODR-PadEnc for new data:

   UDP PAD Request version 1 - message format
   | version (1 byte) |
   | 0x1              |

ODR-PadEnc prepares the PAD block and answers with a UDP reply message:

   UDP PAD Reply version 1 - message format
   | version (1 byte) | PAD data (N bytes) |
   | 0x1              |       <PAD>        |

TODO:

  * Define the order of bytes for the PAD data
  * Endianness of multi-byte values (none yet)
  * Do we want to add a type field with a different value for request and reply?