Improve python gen_protos script

Issues that need to be solved:

- doesn't print output to shell when run during `python setup.py`
- runs futurize twice when grcpio-tools is not present and needs to be installed
- does not account for the case where there are more output files than .proto files (just keeps triggering out-of-date rebuilds due to timestamp of orphaned output file)
- plus it would be nice for debugging if the script printed why it is regenerating pb2 files, because there are a number of possible reasons

Imported from Jira [BEAM-8238](https://issues.apache.org/jira/browse/BEAM-8238). Original Jira may contain additional context.
Reported by: chadrik.