Introduce a DB and pipeline structure specs

[anadahz]

This issue is about introducing a DB and pipeline structure specification.

With so many changes and new features it's hard to track and understand the DB and structure specifications. Tracking the changes (with a versioning scheme) as in all other OONI components is very helpful for data analysis --especially when dealing with older OONI reports.

Currently one can guess the structure of the DB by looking into the pipeline repository and comments in code. I add below an non exhaustive list of relevant documentation sources of the pipeline:

• https://github.com/ooni/pipeline/tree/master/af/oometa
• https://github.com/ooni/pipeline/tree/master/af/fastpath
• https://github.com/ooni/pipeline/tree/master/af/shovel
• https://github.com/ooni/pipeline/tree/master/docs
• https://github.com/ooni/sysadmin/blob/master/docs/metadb-sharing.md

[hellais]

The documentation for the new pipeline can be found here: https://ooni.github.io/pipeline/README.html