Custom framing for UDP

[Bathtor]

Currently (as of #75 ) UDP uses the same framing header as TCP packets, even though much of it is unused on the UDP path.

While it is easier for maintenance to keep the the same, it does waste between 9 and 4 bytes (depending on what we want to keep) of previous space in a UDP packet.

This issue should be considered a reminder to re-evaluate UDP framing in the future.

[adamhass]

Move frame-headers to be handled entirely by the NetworkThread.