node: rename net.NodeSocket and http.NodeAgent

- rename net.NodeSocket interface to net.Socket
- rename http.NodeAgent interface to net.Agent

I understand that it's breaking change for some users of DefinitelyTyped. However it's important to keep the consistency. Why do we have `dgram.Socket`, but `net.NodeSocket`?

I propose to develop a naming convention. For instance, use prefix `Node` only for all global interfaces.