Allow the server to choose a target socket when sending event

[flowersinthesand]

When a physical connection is shared by multiple tabs, because the server can't know there are multiple sockets, they all receives same event. For the true sharing not just synchronizing, it's needed for the server to choose a target socket.

This makes the server implementation more complex and requires another reserved event in the server, but I expect it will be helpful to advanced users.

https://groups.google.com/d/msg/socket_io/7lwKJqyNrvk/indTVDqmqyIJ