Refactor mailing tasks to allow custom queue components

[SDKiller]

Current cron-powered way is hardcoded and does not allow alternatives.

For example, I have RabbitMQ and want to use it rather then storing the whole email html in database and running cron jobs.

IMO, there could be interface for queue component and config option to set it.
And I'd prefer to put in queue email data as array, not entire html.

[tomaszkane]

Sounds reasonable.

[bizley]

Agree.