attempt at simplifying thtp channels even further

After attempting to implement thtp, here's some proposals for simplifying it quite a bit (cc @dchote):

* no size is included, any known size should be in the included THTP packet headers as content-length
* one channel is always one request, no re-using of channels, therefore use "end":true instead of a "done"
* either side can start sending the fragmented THTP request/response immediately (don't need to wait)
* the included packet should be parsed as soon as possible to get the headers, and the body can then be streamed

I'm not done implementing it at [telehash-thtp](https://github.com/quartzjer/telehash-thtp) yet, so there'll probably be more thoughts here :)