better cylc set --output logging

[hjoliver]

hjoliver#47 (comment)

From @oliver-sanders:

If you set multiple outputs on a task, then they will be set one by one. After each output is set, tasks may spawn, have their runahead status changed or be queued. This makes the logs a bit hard to read as log entries relating to the outputs being set are dispersed amongst log entries from other activities.
@hjoliver, would it be possible (follow-on work) to do all the output setting first, and delay the downstream consequences until the end of the routine to avoid this?

The problem stems from the fact that output completion follows the same code paths (from the task events manager) for both natural and manual completion. That's partly historical (the set-outputs command has been around for a while) and partly because we want most of the same consequences to follow from natural and manual output completion: task events, task completion, state change, and spawning of children.

It might be better to more clearly separate the handling of natural and manual output completion.