udp / kcp support

[untoreh]

the tunnel currently can forward udp.
is it worth using kcp-go lib ? probably not because it would just add another listener to tunnel traffic internally to the udp listener itself, so for using kcp an external kcp tunnel can be forwarded to the tcsplitter udp listener.
It makes more sense to implement error correction directly with the vivint/infactious lib.

[untoreh]

udp tunneling should be made smarter anyway because currently:

• every packet is sent over his own tcp connection, they should be aggregated to saturate payload size
• a udp packet cannot be larger than the payload size, should bigger packets be handled? Or should this be a notice for usage?

[untoreh]

current udp implementation buffers packets but data is dumped without re packeting, most udp apps rely on each packet having the right data so each shot that buffers multiple packets should carry the info to re packeting each carried payload