Fixes #22888 - remove code around dynflow db_pool_size

Remove the code in favor of the Dynfow, where we can handle all the
logic at one place.

Fixes #18422 - Expose dynflow worker pool size in config

Fixes #18618 - Use Dynflow as an ActiveJob backend

This commit initializes the Dynflow world in a very similar manner to
foreman-tasks (if a Dynflow world is available from foreman-tasks, we
use that one).