fix Producer::SQLite::batch_alter_table rename field

rename_field is ignored by Producer::SQLite and the existing tests have:

  SELECT ... physical_description FROM person"

which should be:

  SELECT ... description FROM person"

Patches address this and try to make some of the code (mostly inside batch_alter_table) more readable.
No need for the if here, the for loop will just execute zero times if the array is empty.