Write a UTP-based channel implementation

Steven has a "utp" branch in https://gitweb.torproject.org/sjm217/tor.git that begins to implement a UTP-based transport. It requires a tweaked libutp with patches from https://github.com/sjmurdoch/libutp

It is probably close to good enough for initial performance testing. This ticket is about making it good enough to merge, if the initial simulations and testing are promising.