Add an idle timeout notification mechanism to the TCP socket API

[rdw-software]

Similar to timeout events in NodeJS (and Luvit), this would allow setting an idle timeout, which could trigger an TCP_SOCKET_IDLE event. I'm not sure if this is really that useful and it's very easy to implement when needed, so I've left it out of the API I came up with for #61 in order to keep things as simple as possible.