Converted arrays to objects

[darrachequesne]

To be merged after

• Issue Latency/Load/Overhead caused by integer indexed arrays indexOf/splice/shift etc socket.io#2199
• PR converted arrays to objects socket.io#2239

[kapouer]

Hello, changing the broadcast API (from arrays to objects) doesn't improve performance, breaks all other adapters, and it is possible to not change the API and yet change the internal self.rooms to an object.

[darrachequesne]

Hi! After second thoughts, I guess you're right, since rooms and except contains a priori not many elements (and the change of rooms -- and not _rooms -- from array to object is indeed possible without this).
Thanks! I close this now.