improving/filling out the tcp interface, proper closing

 * Added echo-server stresser bash script to the repository
 * Moved to using a handler queue for connections to the tcp server
  Instead of 
``` 
    tcpServer_ = new Tcp.Server(endpoint, function);
```
  You now write: 
```
    tcpServer_ = new Tcp.Server(endpoint);
      tcpServer_.connectionsQueue.setSyncHandler(
          makeTcpToRtcSession_);
```
  And of course you can do whatever you want with the handler queue, as usual. 
 * Added `onceListening` and `onceShutdown` promises. 
 * Added isListening an isShutdown synchronous checking functions. 
 * Improved comments
 * Fixed closing of freedom's tcpsocket interfaces
 * Commented out logging statements

Fixes https://github.com/uProxy/uproxy/issues/489 and https://github.com/uProxy/uproxy/issues/485 
```js
Fulfills once the server has server has successfully bound to a port
and is accepting connections. Rejects if there is any error.
```

And mention that `listen()` returns this.