|SocketIORunnable| is a runnable class template that holds a
reference to a arbitrary Socket I/O object. |SocketIORunnable|
replaces |UnixSocketImplRunnable|, which only handles objects of
type |UnixSocketImpl|.
|SocketConsumerBase| handles connection state for |UnixSocketConsumer|
and its derived classes. Implementing classes must override a number of
virtual methods, to handle notifications about changes to the state of
the connection.
