Connections now resolve with a ConnectionInterface

[clue]

This PR changes the resolution value from Stream (which implements the DuplexStreamInterface) to an instance implementing the new ConnectionInterface, which in turn implements the DuplexStreamInterface.

In other words: Each ConnectorInterface now has a connect() method that resolves to a ConnectionInterface.

Because the ConnectionInterface implements the same base interface DuplexStreamInterface, it behaves just like a normal stream resource, so consumer code can simply use the new typehint and continue to work unchanged.

The ConnectionInterface also implements two new methods which are not available for the underlying stream:

• getRemoteAddress(): ?string
• getLocalAddress(): ?string

Resolves / closes#44
Refs reactphp/reactphp#199
Refs reactphp/socket#65
Refs reactphp/socket#68

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Note that this React\SocketClient\ConnectionInterface is somewhat similar to the React\Socket\ConnectionInterface (client context vs server context but otherwise identical methods).

It is believed this is a good thing currently, because we aim to avoid inter-package dependencies.

In the future, we will look into merging these into a single package again, in order to avoid confusion. This debate is best left out of this PR, be assured I'll file new tickets and link here if this ticket is accepted 👍

[clue]

In the future, we will look into merging these into a single package again, in order to avoid confusion. This debate is best left out of this PR, be assured I'll file new tickets and link here if this ticket is accepted 👍

See #86 for future plans to merge this into the Socket component.