Refactor so a thread isn't spawned for each sent message.

When creating a TcpSender, a helper thread is spawned which receives messages to send. This way, sending is lighter-weight and still nonblocking while maintaining a predictable order for received messages.
Merge branch 'master' of https://github.com/tikue/superchan into tikue-master
Merge branch 'tikue-master'
Make `std::comm` usages more explicit.