AIP-44 Migrate DagFileProcessor.execute_callbacks to Internal API.

[mhenc]

Note that Callback execution must be still done in DagProcessor

[vincbeck]

I'd like to work on this one

[vincbeck]

I dont think the method execute_callbacks needs to be converted itself but rather the submethods _execute_task_callbacks and _execute_dag_callbacks. What do you think? The submethod manage_slas has already its own issue

[mhenc]

Right, I should have been more descriptive:
The problem about this task is that executing the callback itself must be still done in the DagProcessor (not Internal API) component.
One the other hand all requests to DB must be done through Internal API. E.g. _execute_task_callbacks first read data from DB then call ti.handle_failure(which unfortunatelly also tasks session in the arguments and ameks some additional DB calls.).
This all must be untangled - even with refactoring of some methods/introducing new