Embed UnimplementedFooService in every service

[dfawley]

This should include: examples, tests, benchmarks, and most importantly: reflection, interop, channelz, health, etc.

[dfawley]

We will leave reflection, health, channelz, interop, etc, alone in order to notice when new methods are added. We wish to always implement all methods and can tolerate build breakages.

This is only acceptable because our repo is the canonical source of the generated proto files for golang. If the protos were maintained elsewhere, a build breakage could occur when users referenced a later version of the protos than our repo.