* Cleaning/simplifying some buffer bits
1. NACKs are always inserted in order. So, get rid of
bunch of out-of-order handling in there and simplify.
2. For now, removing triggering a key frame from NACKs.
Let subs drive it.
3. Move to 16-bit sequence numbers except for receiver
report handling. Simplify bits about unwrapping sequence
number on all packets.
4. Remove unused code.
* remove unused field