Access to query strings (GET params) in user-defined endpoint handlers

[SockJS (the client side) allows adding query strings to the endpoint URL.](https://github.com/sockjs/sockjs-client/issues/72)
Here, currently there is no way to access them (e.g., `request.GET`) inside user-defined endpoint handler coroutines. I'd like to access them! :eyes:

In the code, sockjs seems to invoke the user-defined endpoint handler in `SessionManager.acquire()` and `_acquire()` methods, which is called by transports' `process()` coroutines. `SockJSRoute` and transport subclasses have access to the aiohttp's request object but it is not passed to the user-defined handler via `acquire` method chains.

Maybe it is not a good idea to pass the whole request object to user-defined handlers due to design principles such as "separation of concern", but still I think it would be nice to have access to GET parameters.