Better debugging support

[amazeika]

Problem

Debugging any scheduler job is not an easy task at the moment because we need to either wait for it to execute a job when the time comes or we have to trick the component to execute those jobs at demand (this is quite tedious).

Proposal

1. Append XDEBUG_SESSION_START=1 into the scheduler URL so that these requests may be debugged. This can be done when debugging is enabled in the site.
2. Make use of a configurable constant (or any other configurable property/variable) to force scheduler to run every single time regardless of its previous runs.