Add scheduled tasks support by the way of Swoole\Timer

[kenashkov]

These are to be started in a dedicated TaskWorker. This worker may distribute work to other TaskWorkers.

[kenashkov]

Add a check at code startup - if there are scheduled tasks but no TaskWorkers configured throw an error.

[kenashkov]

Add a separate package for parsing scheduled tasks definitions - one of these should support the standard cron format.

[kenashkov]

A new package is created: azonmedia/cron :

• https://packagist.org/packages/azonmedia/cron
• https://github.com/AzonMedia/cron