use the request context with net/http handler

[groob]

When the grpc server is used as a net/http handler the request context is being discarded. This breaks logging and other uses where grpc and http are served on the same port in a Go application.

This PR adds the request context to the server transport. I see that grpc is maintaining compatibility down to 1.6 so I'm happy to make the context changes backwards compatible. Looking to get some initial feedback first.

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[groob]

CLA signed.

[menghanl]

This doesn't work in go1.6 because Context() doesn't exist. We had similar problems like this, so there are go16.go and go17.go.

One solution would be to add a contextFromRequest() function, which returns background for go1.6, and req.Context() for >=go1.7.

[groob]

Yes, @menghanl I'm willing to update the PR to make it compatible with 1.6+ by adding that function. That's the feedback I was looking for :)

[groob]

Updated to account for 1.6 vs 1.7+

[menghanl]

Thanks for the fix.
LGTM.