A mapping from still-bonded eras to the first session index of that era.
Must contains information for eras for the range: [active_era - bounding_duration; active_era]
[active_era - bounding_duration; active_era]
A mapping from still-bonded eras to the first session index of that era.
Must contains information for eras for the range:
[active_era - bounding_duration; active_era]