upgrade to protobuf 4

[pjfanning]

• We've been getting build failures in this repo when we try upgrading protoc and/or protobuf-java to v4
• We have dependencies on grpc-java and scalapb which both still default to protobuf-java 3.
• scalapb 1.0.0 alpha support protobuf 4 #610 shows that we can get some of the way to supporting protobuf 4 but there are issues