We merge them into a single interface and support multiple of them rather than
just one.
This will allow us to stack handlers with different responsabilities and extract
some core logic (like tracing) out to a handler where it belongs.
Change-Id: I6aab92138550c5062fcb1bed86171e0850d1eb38
Reviewed-on: https://go-review.googlesource.com/c/tools/+/185879
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
