The `options` that were being passed in before here are specific to a
single request, which kinda defeats the purpose of using an Agent in the
first place.
On a worse note, these `options` have not yet been "processed" by the
`http.ClientRequest` class, so if `port: null` is set (like it is as the
result of a `url.parse()` call), then they take preference over the
processed values since the agent's "options" get mixed in last in the
`createSocket()` function.
Fixes#6197.
Fixes#6199.
Closes#6231.
There are some agent subclasses using this today.
Despite the addRequest function being undocumented internal API, it's
easy enough to just support the old signature for backwards
compatibility.
Instead of destroying sockets when there are no pending requests, put
them in a freeSockets list, and unref() them so that they do not keep
the event loop open.
Also, set the default max sockets to Infinity, to prevent the awful
surprising deadlocks that happen when more connections are made.
