Activity committments and activity log based on epochs

[daria305]

Description of change

Solves #2285

• Create and save to snapshot activity commitments
• Change ActivityLog to use epochs instead of time
• Update unit tests

Type of change

Choose a type of change, and delete any options that are not relevant.

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

How the change has been tested

unit tests, integration tests

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• [ ] My code follows the contribution 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