[feature request] Verify Compiled Protos on Travis

[cfromknecht]

As pointed out by @xsb in #2312, different authors may have different proto versions installed when doing development. To ensure all PRs use the correct of protoc and plugins, we should move to have Travis verify that the output is correct, and fail the build otherwise.

Steps to Completion

1. Have travis build and install the protobuf dependencies specified here.
2. Cache the protobuf and plugin source directories in the travis build cache, so that it doesn't need to be recompiled from scratch on every build.
3. Add a new make target to Makefile, e.g. rpc-check, that first runs the rpc target, and then asserts that the source tree is clean. This can be done by using $(shell git describe --dirty) and asserting that the output doesn't contain the string "dirty"
4. Add the rpc-check target as a dependency to travis-race, travis-itest, and travis-cover, preferably after lint so that the more expensive operations are done only if the protos are sound.

[johng]

Would it not be better to not have the generated protobuf files in source control? Instead the generation would be part of the build step.

[cfromknecht]

@johng the protos should still be checked into source. The intent is that by recompiling them in Travis, we should be able to tell if they’re correct if doesn’t leave the source tree in a dirty state

I started working on this in #3012 and #3010

[johng]

@johng the protos should still be checked into source

Why?

[cfromknecht]

@johng so that installing/building grpc and the related plugins isn’t required to compile lncli