refactor: Unify logic for generating Redis keys

[morgsmccauley]

Both the IndexerState and IndexerConfig structs are used individually to access Redis, and therefore have duplicate code for generating the required keys. This is problematic for the following reasons:

1. Redis access is limited to these structs, anything else requiring access needs to duplicate the logic again
2. Updates need to be reflected across both structs, creating more room for error

This PR creates a common Redis KeyProvider interface/trait for accessing Indexer Redis data/keys. Now the required structs, i.e. IndexerState and IndexerConfig, can implement this trait, and automatically get access to the methods which generate the keys.

Also did some minor clean up, removing the old Coordinator migration code.