This is the first step to upgrade grpc dependency to the latest version.
- Patch protobuf 3.6.1. and grpc 1.15.1 to fix build errors when using the latest Bazel version (3.4.1).
- Add grpc/tests:grpc_test. One can kick off tests in Bazel by calling `bazel test grpc/tests/...`.
- Add missing build targets in tests/BUILD in order to support grpc/tests/BUILD
Shengzhe