Reduce code duplication between GSIs and tables

[sebdah]

Currently the project has a separate set of core code for GSIs and tables. That is to a large extent unnecessary and makes the code harder to maintain. The reason for the code separation is large due to:

• Different logging output (minor)
• Different way to fetch configuration values

It would be great to merge all logic for scaling etc and have an interface that both GSIs and tables could use.

This would increase project testability and maintainability a lot.