Refactor Datastore-related task queries.

[isoos]

• Creates new methods inside _TaskDataAccess (listFinishedSince, selectSomePending) following the previous query refactor patterns to make the eventual SQL-migration simpler.
• Updated updatePackageStateWithPendingVersions to take a package name instead of PackageState.
• Updated updatePackageStateWithPendingVersions to return the previous state's versions Map in case we need to revert it.
• most of the schedule method's change is coming from the query method refactor + the subsequent indent, however, there is a noteworthy update: instead of a fully asynchronous scheduleMicrotask(() async {, we now use await Future.microtask, I think it fits the method's Future.wait wrapper better.
• The related transactions will be refactored in a subsequent PR.