Pass request context to frame observer

[martin-sucha]

This allows the frame observer to access the context passed to gocql.

[medasx]

non-blocking: I wonder whether it would be possible without passing context to callReq. As I see it would require many more changes.

[martin-sucha]

Good point. We can't easily get rid of frameParseObserver.reqCtx (that would require moving all the frame parsing code to one place), but we don't need to store the context in callReq. Thanks!

[medasx]

Thanks!