Trying to speed up tests

[stanley-cheung]

Trying to speed up tests:

• for tests triggered by pull requests (i.e. basic tests and interop tests), if possible, avoid having to compile grpc and protobuf from scratch
• use bazel to compile the protoc-gen-grpc-web plugin instead of make plugin, seems much faster
• for master branch continuous integration testing (i.e. once every X hours), that can take as long as it needs

other changes:

• move bazel and buildifier tests into some docker image to minimize changes to the host's system, plus the benefit of being able to reuse bazel