refactoring of GLRD codebase

[yeoldegrove]

What would you like to be added:

As suggested in #3 the code base could be refactored.

Why is this needed:

• reuse variables/constants
• reuse code where possible
• rethink if additional Classes can be introduced
• rethink if main can be separated again
• rethink how schema updates and the affected production data are processed (glrd-update)

[NotTheEvilOne]

Here's a quick list of suggestions:

• Restructure Python packages to contain __main__.py files. That would incorporate splitting __main__() and parameter related definition from business logic. See https://docs.python.org/3/library/__main__.html
• Maybe add additional Python packages. E.g. src/glrd (takes code from glrd_query.py, util.py renamed and splitted to constants.py and parser.py), src/glrd/scheme taking SCHEME from glrd_manage.py and logic from glrd_update.py and src/glrd/releases taking the remaining content of glrd_manage.py
• Further splitting S3 related actions from GitHub ones