- Add migration helpers for managing NOT NULL
  constraints (add, validate, remove, check if exists)
- Update migration helpers to use add_not_null_constraint to
  enforce NOT NULL for existing columns instead of change_column_null
- Update the disable_statement_timeout migration helper to
  allow calling it multiple times, even from inside a block
  of another disable_statement_timeout, without resetting the
  timout before the outer block finishes
- Update the Database Guides with a reference to add_not_null_constraint