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()`.

Adds a test to ensure that BuildStream alerts the user of a skipped push
when the remote already has the artifact cached.

This removes the timed_activity for an element _push action. This is
unnecessary as the job is already being timed elsewhere.

This changes the info phrase for when an artifact is pulled from a
remote server.

Both pulling and pushing INFO messages are now status messages.

Calls to the messaging API through `self.context.message()` have now
been switched to `element.info`.