This PR implements a simple getLocalAddress() method which complements the existing getRemoteAddress() method already present on the ConnectionInterface.

This can be useful if your Server instance is listening on multiple interfaces (e.g. using the address 0.0.0.0). You can use this method to find out which interface actually accepted this connection (such as a public or local interface).

Adding this method is a new feature from a consumer perspective. Marking this as "BC break" in case any consumers of this package implement this interface (this should not affect most users, aka. normal usage).

Refs reactphp/reactphp#199 and #65