Connection is a server side incoming connection

It MUST NOT be used in a client side context.

This has caused several issues already, so we should update our documentation accordingly.

> The `React\Socket` namespace is part of the `react/socket` package which is a "Library for building an evented socket server". Hence the `React\Socket\Connection` should be used for an *incoming connection* and not for an *outgoing* one.

Refs #4, reactphp/stream#11 and reactphp/dns#11.