Implemented system-tasks

[wachterjohannes]

This PR introduces the feature of system tasks. You can now configure predefined tasks in you config tree:

task:
    system_tasks:
        my_task:
            enabled: true
            handler_class: 'AppBundle\Handler\TestHandler'
            workload: 'my workload'
            interval: '@daily'
        ...

Related PR: php-task/php-task#31 php-task/docs#4

TODO

• docs

[alexander-schranz]

Upgrade for the database is missing

[alexander-schranz]

Old system tasks are not removed from executions.

[danrot]

Wouldn't false be a better default? Or does it still work if the three settings below are not set?

[wachterjohannes]

this is a prototype which can be extended. when i add a system-task it should be enabled by default. if i want to keep the config but want to disable it you can set the enabled flag.

[danrot]

If the old tasks continue to work as until now, and no configuration has to be adapted (I already commented a line which is suspicious) I don't see how that should break Sulu.

[wachterjohannes]

@danrot problem is that you have to update the schema of the table

[danrot]

Looks like you are merging it into develop, so it will only come with the next minor release? Should be fine in my opinion.

[wachterjohannes]

@danrot yes it will go into 1.1.0