Handle the readyRead() signal in RemotePeer instead of LegacyPeer

That way, the RemotePeer subclasses don't all need to connect to the
signal themselves; also, we can make sure in a central place that
socket data is processed even the socket was handed over to the peer
with unhandled data still in its buffer.

Instead of handling the signal directly, RemotePeer subclasses need
to implement the onSocketDataAvailable() method.