Now generating the extension in place once all the data is already
in the right place.

Adding a 'D' byte to signal the DRED experiment, along with a version
number byte. This entire commit will be reverted once DRED is finalized
and given a non-experimental extension number.

Fixes "FEC buffer full" issue

Properly re-initialize DRED (only) when needed

Probably still has many bugs

Now only running the decoder on "received" packets