DataLayer: update multiple data stores in a single call to batch_update

[fizpawiz]

Most databases have a notion of a "database transaction" where multiple tables can be updated at once, and on commit, the changes to all tables either succeed together or fail together. This avoids data irregularities where one table got updated and another did not, leaving the database in a nonsensical state.

I would like the same feature for Datalayer, where multiple data stores can be updated in a single call to batch_update. The spends of all the singletons would be interlocked with announcements such that the entire update is either successful, or unsuccessful, and it is not possible for one data store to be updated without also updating the other(s).