Change regenerate.sh to use cmd/protoc-gen-go-grpc and rerun it.
This splits the gRPC service definitions into separate files, but
otherwise has no changes.
Replace various //go:generate lines and regenerate.sh scripts with a
single, top-level regenerate.sh that regenerates all .pb.go files.
Placing generation in a single script ensures that all files are
generated with similar parameters. The new regenerate.sh uses the
protoc-gen-go version defined in test/tools/go.mod and automatically
handles new .proto files as they are added.
Do some minor refactoring on .proto files: Every file now has a
go_package option (which will be required by a future version of the
code generator), and file imports are all relative to the repository
root.
Also, two fixes:
- Fix long-standing `.travis.yml` bug where `VET_SKIP_PROTO` was not `export`ed (so not seen by `vet.sh`).
- Update `vet.sh` to work with new `goimports -l` that does not print a `:` after filenames.
