Redo/Refactor/Fix metadata logic

[ethervoid]

Related #294

During the related task I've found some problems with the metadata tasks:

• Some tasks like ColumnTask is not idempotent and does operations that are not recommended by Luigi like:
  • Have logic in the complete method
  • Call and get the output objects in the run method
  • Call complete from output method
  • They're not idempotent because they don't have a clear output
• Also I've found that the ORM model is more complex than needed so we probably have to check if we can simplify it
• We have more tasks without an idempotent output so we have to fix that too

[ethervoid]

Meeting summary and conclusions

Yesterday we had a meeting to discuss what we should do with this ticket and if we block it or continue working on it.

The conclusion was:

• Block the ticket and stop spending time on it because we have to fix first some other issues like the metadata one (that is covered in this ticket)
• Create and be aware of this ticket but don't start working on it immediately because the problem is not a blocking one.

[juanignaciosl]

As a specific conclusion of "be aware of this ticket", we agreed on considering creating a simpler task class hierarchy when we tackle the "declarative ETL" task in the future.