MigrationInterface::isComplete() renamed to allRowsProcessed()

The migration entity's isComplete() method was incorrectly implemented - it was meant to determine whether the migration had been fully processed, but it was looking for a "migration result" of COMPLETE (which reflects the completion of a migration operation, not complete processing of the migration). The method was changed to properly check that all source rows of the migration have been processed, and has been renamed to allRowsProcessed() to clarify what it's doing and avoid the overloaded word "complete".

As a result of this change, the only remaining use case for placing operation results in a key-value store is to communicate the result to be returned when interrupting a migration. Therefore, we've removed the setMigrationResult() method and renamed getMigrationResult() to getInterruptionResult() to clarify its purpose.