add keep-alive support to socket_listener & socket_writer

[phemmer]

Closes #2635

Unfortunately I cannot think of any way to add unit tests here. There's no ability to query a socket for its current keep-alive setting. The only way to trigger a socket shut down due to keep-alive is to kill a host, or sever the network. And the only way to observe a keep-alive probe is a packet capture.
{rock} (us) {hard place}

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[phemmer]

so
    much
        nesting

The only way I can think of to flatten this out a bit would be to break the code out into a function and use returns. Not sure how I feel about that.

[danielnelson]

I think you could probably do something like func setKeepAlive(*net.TCPConn, time.Duration) error, might have to add the service address context to the error outside the function. Would probably clean up the else branch when a duration is non-zero. Let me know if you want to make changes or not.

[phemmer]

Yeah, that was along the lines of the idea, just thought it might be a little odd breaking it out. But I went and implemented it (and pushed to PR) and I think it's clean.