Recommend to backfill existing int IDs on backfilling

Why?

On backfilling a table using backfill_conversion_of_integer_to_bigint helper, we do a full table scan. We will eventually convert all existing integer IDs to bigint (#465805 (closed)), we can make use of the full table scan when done to combine updating other integer IDs as well. This will save us a lot of time (will take months for few large tables) spent on backfilling migrations.

What?

Raises an exception from initialize_conversion_of_integer_to_bigint method if there are any pending integer IDs in the table, that can be initialized to be converted to bigint.

More details can be found in here.