deprecate 1.6 and embrace (*http.Request).Context by default

[tmc]

I'm opening this to gather feedback. With 1.8's release I think we should now talk about removing 1.6 support by default and allow per-http request. This will enable things like tracing over gateway calls.

[achew22]

+1, I think following the release cycle of Go closely will help keep down the clutter. This will represent a breaking change. Does that mean a 2.x series?

[tmc]

while I'm skeptical of semver, yes I think so. We should get another 1.x release out before cutting a 2.x

[achew22]

This is now available by a flag.