Rework Skipped usage

The SKIPPED message type is now used to indicate the end of a task which
was successful without having to perform the given task.

This overhauls the use of `Queue.done()` and therefore queues do not
need to provide a processed/skipped return value from `done()`. Instead
this is replaced with the action of raising a `SkipJob` exception from
within `Queue.process()`.