Race condition may lead to an incorrect count in MigrationHelpers#update_column_in_batches

This occurred in gitlab-com/operations#355 today. What can happen:

1. SELECT COUNT(*) AS count FROM projects is run
2. total is set to N
3. A new project is created
4. The update happens for all the other projects

The new project isn't marked with the default value. If change_column_null attempts to run, it will fail, leading to a failed migration.

/cc: @pcarranza, @rspeicher, @twk3, @tmaczukin