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
flatbuffers project currently depends on an old version of GRPC library and has known issues working with the latest version (issue #5836). Therefore, a patch file is created and put under bazel/ directory before supporting the latest GRPC version.