Previously it matched the behavior of standard ActiveRecord after_commit callbacks. They do not work well within `joinable: false` nested transactions. Now `DB.after_commit` callbacks will only be run when the outermost transaction has been committed.
Tests always run inside transactions, so this also introduces some logic to run callbacks once the test-wrapping transaction is reached.
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
