grpc plugin: Make RPC call implementation more modular.

grpc plugin: Make RPC call implementation more modular.

Use a template class for the RPC call object implementation to handle all
common functionality. The actual RPC implementation has been moved into
overloaded functions, one for each RPC. This approach only works as long as
the request and response type pairs are unique per call. That's common
practice, though, and in case there's an exception, it would have to fall back
to the previous approach (one class per call).