Hidding session immediatly after stop is called,

This should make appium more stable, The closing process takes time, and commands were not prevented from coming in. Now they will be blocked by [sessionBeforeFilter](https://github.com/appium/appium/blob/master/lib/server/controller.js#L51).

@jlipps can you check. @0x1mason that fixes the timeout issue.

Also noticed that sessionBeforeFilter returns 404 which I think is inaccurate, but will open another issue for that, should be part of a minor release.