Flags for deferred tasks

[sveneberth]

The current implementation of deferred tasks tries always to get the current user and run the task with the associated credentials.
It would be nice to have the chance to control this behavior. E.g. to have the possibility to run a task anonym.
My proposal is to introduce an optional parameter user with this you can submit a user or None to run with specific or without any credentials.
Maybe this could be also interesting for the language.