Change the data type of the fileID map in the StateJournal to "int".

[deniadelcid]

Background: https://github.com/googleinterns/step224-2020/pull/57/files#r490811876

Specific reasoning: ''Using int32 instead of int (which is typically int64 on modern systems) doesn't save much space, especially if there are only that few entries in the map. It's a premature optimization. Just use int everywhere."

[evanSpendlove]

Use strconv.ParseInt instead of strconv.Atoi. ParseInt returns int64, so you don't have to convert afterwards.