Allow customising of queries used for `update_column_in_batches`
This MR makes two changes to `add_column_with_default` and `update_column_in_batches`:
1. `add_column_with_default` no longer wraps the entire set of updates in a single transaction, preventing any locks from sticking around for the duration of the entire transaction
2. `update_column_in_batches` now takes a block which can be used to customise the queries. This uses Arel as messing with raw SQL strings is a total pain
In !4381 there's a need for updating existing rows/columns in a table in batches using a custom `WHERE` condition. Without the changes in this MR this would not be possible.
See merge request !4680
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.