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• does not account for the case where there are more output files than .proto files (just keeps triggering out-of-date rebuilds due to timestamp of orphaned output file)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