[server] introduce a scheduler class to run repository "looping tasks" We just use the sched module from the standard library and introduce a tiny Python2/3 compatibility layer (more for convenience actually). The "looping" aspect of tasks (previously in LoopTask class) is re-implemeted as a `schedule_periodic_task` function. This is a reasonably thin layer as compared to LoopTask/TasksManager classes. Only the "restart" aspect of LoopTask is no longer present as I'm not sure it's worth keeping. The advantage of using this (in addition to eventually dropping our custom code) is that this scheduler class provides a `run` method that blocks the process while running tasks in its queue. So we can rely on this to have a 'scheduler' ctl command (see forthcoming patch) that would only run "looping tasks" without having to implement the "blocking" aspect ourself. Related to #17057223.

3.25 (UNRELEASED)
=================

New features
------------

* A new option `connections-pooler-enabled` (default yes) has been added. This
  allow to switch off internal connection pooling for use with others poolers
  such as pgbouncer_.

.. _pgbouncer: https://pgbouncer.github.io/


* A new way to declare workflows as simple data structure (dict/list) has been
  introduced. Respective utility functions live in ``cubicweb.wfutils``
  module. This handles both the creation and migration of workflows.