This changes update_column_in_batches to ensure it always updates all
rows now. These changes also allow for an extra SELECT query to be
removed, nor does it use the row count for determining offsets and the
likes; instead it's only used to determine the batch size.
This ensures that whatever locks are acquired aren't held onto until the
end of the transaction (= after _all_ rows have been updated). Timing
wise there's also no difference between using a transaction and not
using one.
By passing a block to update_column_in_batches() one can now customize
the queries executed. This in turn can be used to only update a specific
set of rows instead of simply all the rows in the table.
Instead of updating a fixed number of rows (based on the amount of rows
available at the start of the update) the method
"update_column_in_batches" will now continue updating rows until it runs
out of rows to process.
For a table with a high rate of inserts this may result in the migration
taking quite some time. However, the alternative is not all rows being
updated or the "change_column_null" method raising an error due to there
being NULL values.
These helpers can be used to perform migrations without taking down the
entire application.
For example, the method "add_column_with_default" can be used to add a
new column with a default value without locking the entire table.
