Simplify execute_migration method in Elastic::MigrationWorker

Currently we check whether the migration is persisted? but should look into whether using started? could work better (See discussion thread in linked MR for more info).

The following discussion from !47819 (merged) should be addressed:

• @terrichu started a discussion: (+1 comment)

  I realized during manual testing that batched needs to be checked here since batched migrations will be persisted & not completed. I also adjusted the specs to include cases for this.