Session ID in responses

Hi,

The original JSON wire protocol used by selenium webdriver included the
session id in every response. I think we should do the same thing in the
W3C webdriver protocol.

The advantage this offers is that it means that there's no need for an
intermediary node to pair up requests and responses, instead just keeping
track of the various sessions being run (opening up some nice queue-based
possible implementations that would otherwise need additional
book-keeping). From an end-node point of view, the session ID is known
anyway[1], so adding this to each response shouldn't be onerous.

Thoughts?

Simon

[1] I assume. I'd need to actually read more code to be certain :)