Rename protobuf messages

[danieljanes]

Summary of changes:

• Change package name in transport.proto from flower.transport to flwr.proto to match the Python package name. This package name will probably change one more time in the future if we decide to move the .proto files into the Python package structure, this change is only there to align the Python and ProtoBuf package names.
• Use Ins/Res more consistently, e.g., rename GetParameters to GetParametersIns
• Use imperative naming, e.g., rename ParametersRes to GetParametersRes