Label all tracer events with a severity that the Consensus Team decided

Current plan:

* Create a type class that labels an event with severity. For at least some events, Consensus Team is best positioned to decide this label.

* To avoid confusion/ambiguity -- ie for consistency -- we'll assign a severity to every event, not just the ones that we're best positioned to do so.

* Add a wrapper type to our tracers at the boundary (ie in StdRunNodeArgs) by invoking the class method.

* If an event's severity depends on dynamic information, that event must carry enough info to decide it.

* The clients can opt-out of these, or incrementally incorporate the new severity level, by discarding the wrapper type at any stage. Ultimately it's their call what to do with this label. Ideally we'd reach mutual agreement on what it should be.