Improve updating, tracking and indexing in epoch indexer

[pharr117]

The epoch indexer is still a bit underdeveloped and requires manual tracking and intervention to get rewards distributions. We want to improve the workflow like so:

1. Add a new indexed boolean column to the Epochs model, defaulting to false when stored by the update-epochs command
2. During epoch indexing, as a first step:
   • Get the current epoch from the RPC server
   • Compare to our database
   • Gather epochs that are missing
   • Run the epoch indexer for all epochs where indexed == false
3. When an epoch finishes indexing, set indexed = true