net: make Server.prototype.unref() persistent

[cjihrig]

Currently, the unref() method does not remember any state if called before the server's handle has been created. This commit adds state to track calls to ref() and unref().

Related to #7077. Note - a similar fix is possible for Socket.prototype.unref() (which I have code for). However, Socket appears to have additional unref() issues other than persistence.

[tjfontaine]

This looks like it should be targeted at v0.10 as a bug fix, are there other places this is true -- for instance in dgram?

[cjihrig]

Closing in favor of #9115.

@tjfontaine Regarding dgram - the following code prints 'bound' and then exits. Removing the unref() causes the program to not exit.

var dgram = require('dgram');
var socket = dgram.createSocket('udp4');

socket.unref();
socket.bind(8000, function() {
  console.log('bound');
});