Conform the AZSocket.io state to the socket.io spec

According the socket.io spec (https://github.com/LearnBoost/socket.io-spec#anatomy-of-a-socketio-socket) the the possible states for the socket
are `disconnected`, `disconnecting`, `connecting`, `connected`.
When the `[AZSocketIO disconnect]` is called the socket
goes in the state `AZSocketIOStateDisconnecting` while asking to the
transport to effectively close the connection, after that the socket
will be in the `AZSocketIOStateDisconnected` state.

I've also renamed the enum item to be consistent with both the socket.io naming and following the cocoa coding style where the enum item are prefixed with the new type name.