check if the BABE next epoch is already persisted

[EclesioMeloJunior]

Describe the bug

In the dot/digest package we listen for finalized headers and when a new finalized header arrives we execute the function persistBABEDigestsForNextEpoch(finalizedHeader *types.Header). This function retrieves the epoch for the finalized header add 1 and uses the result as the nextEpoch and then call the h.epochState.FinalizeBABENextEpochData(nextEpoch) and h.epochState.FinalizeBABENextConfigData(nextEpoch) in order to persist the next epoch BABE data and config and remove the in memory data.

The problem is that when another header is finalized in the same epoch we call the same functions again but this time they will fail as there is no data to persist in the database.

A possible solution is to check if the nextEpoch already has defined some data in the database and if so we skip the respective persistency step