Allow clients to specify a sessionid when connecting.

[mthomas]

This is a proposal for new functionality.

The intended use case is to allow clients to reconnect using the same sessionid that they originally connected with. This, combined with the RedisStore would allow a socket server to fall out of a cluster, a client to reconnect to another server in the cluster, all while maintaining data associated with the socket.

My proposal would be to modify the Manager.handleHandshake method to pull a sessionid from the query if it exists.

In order to utilize this feature, clients would have to set the sessoinid in the socket.options.query (client).

Any thoughts?

I'll submit a pull request including a new test for your consideration!

[mthomas]

Pull request including new functionality+test: #498