grpc.load is deprecated so we need to switch to proto-loader. This refactor was started in googleapis/gax-nodejs#229 but some more work needs to be done.

Also, I'm planning to rethink the idea of COMMON_PROTOS: since proto-loader supports searching in several directories (includeDirs option), this logic can be offloaded to it.

TODO:

1. Do not use grpc.load.
2. Simplify COMMON_PROTOS logic.
3. Make tests pass.
4. ???
5. PROFIT!