Add conventions for settlement spans for messaging

[pyohannes]

Based on open-telemetry/oteps#220, conventions for "settle" spans should be added. This involves:

• The "settle" operation.
• Directions of when to create a "settle" spans.
• Defining the relationships of the "settle" span to other spans.

It's not the goal to stabilize those together with messaging semantic conventions. We will work towards a "mixed" stability, where "process" and "settle" spans are experimental, but other parts of the semantic conventions are stable.

The motivation behind is to not delay stabilization efforts with discussions around details, while still having confidence that we can stabilize "process" and "settle" spans without breaking other parts.