OGM-605 [MongoDB] Avoid dots in column names & Map composite keys more naturally (not in a nested structure)

This issue addresses:
- associations pointing to composite ids now avoid the nested structure and place the 1st level of columns flat
    e.g. runners: { race_runnerId: {firstname: Emmanuel, lastname: Bernard}, ranking:0 } => runners: { firstname: Emmanuel, lastname: Bernard, ranking:0 }
- dots no longer used even in global and collection per association mode

Pay particular attention to `AssociationCompositeKeyMongoDBFormatTest` as it describes the expected structure.