Add disconnect method and fix up Network class

[zbjornson]

⚠️ (This is based on my master branch, which has my two other PRs merged into it.)

Adds a serf.disconnect method that closes the underlying socket, allowing node to gracefully exit.

Also fixes up the network class a bit:

• Do not call the Network.connect callback more than once. This was being called both after connecting and on any socket errors (the net.on("error"...) handler wasn't actually handling connection errors).
• Properly isolate members -- make new Socket and EventEmitter instances for each instance of Network. Begins to address RPC is a singleton #5.

(Trying to submit small, isolated PRs, but to do the rest of #5 it will be rather invasive.)

[zbjornson]

@normanjoyner any thoughts on these three PRs?

Happy to be a formal contributor if you want to add me to this repo (or transfer it to me). Have a few more changes (including what may be a critical bugfix for large serf clusters) ahead.