Add interval and count to keep-alive

[jameshartig]

We should be able to specify the probe interval and the failure count for keep-alive's on sockets.

Corresponding node pull request: nodejs/node-v0.x-archive#2239

[piscisaureus]

We should be able to specify the probe interval and the failure count for keep-alive's on sockets.

Why is that needed?

[jameshartig]

It is needed because the default probe interval and failure counts set by the OS are too extreme and if you have an application that keeps a lot of sockets open (say a server that hosts a lot of clients over socket connections) you'd want to fine-tune the values to be very low.

[ibc]

I agree. Depending on OS default values is not (always) good.

[jameshartig]

If I make a new pull request pointing at master and add the documentation for count/interval being 0, can it be merged?

[jameshartig]

Updated with Windows support for interval, documentation added that @txdv suggested and targeted master.
#729