Add configurable variables for spent addresses DB

[viossat]

Description

The SPENT_ADDRESSES_DB and SPENT_ADDRESSES_LOG paths are currently hardcoded. This PR makes them configurable similarly to DB_PATH and DB_LOG_PATH.

Fix #1268

Type of change

• Enhancement (a non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[GalRogozinski]

Thanks @viossat

I want to have a quick talk with the IRI team on my comment, #1268 (comment), before we merge.

I just want the process of syncing from local snapshots to be clear