This JIRA is for some simple refactors in the NN:
- refactor the code which writes the seen_txid file in NNStorage into a new "LongContainingFile" utility class. This is useful for the JournalNode to atomically/durably record its last promised epoch
- refactor the interface from FileJournalManager back to StorageDirectory to use a StorageErrorReport interface. This allows FileJournalManager to be used in isolation of a full StorageDirectory.