Sometimes cascading association deletions can cause timeouts due to
an IO issue. Perhaps a model has associations that are destroyed on
deletion which in turn trigger other deletions and this can continue
down a complex tree. Along this tree you may also hit other IO
operations. Such deep deletions can lead to server timeouts while
awaiting completion and really the user may not notice all the
changes on their side immediately making them wait unnecesarially or
worse causing a timeout during the operation.

We now allow associations supporting the `dependent:` key to take `:destroy_async`,
which schedules a background job to destroy associations.

Co-authored-by: Adrianna Chang <adrianna.chang@shopify.com>
Co-authored-by: Rafael Mendonça França <rafael@franca.dev>
Co-authored-by: Cory Gwin @gwincr11 <gwincr11@github.com>