HTTP client: Add maxFreeSockets option

[isaacs]

Currently, sockets that are waiting around in a free state are not limited, except by the max number of sockets that you'll ever open.

So, if you set maxSockets:Infinity, so that you always open up a new one if none are available, then you could have quite a lot of free sockets left hanging around.

Add a maxFreeSockets option, so that you can limit the number of sockets left open in a free/pending state.

[geek]

@isaacs do you remember the reason why its defaulted to 256?

[sudosoul]

@geek this is both late and a guess, but I assume because 256 is a general minimum ulimit hosts have set.