In certain cases, we do not actually promote each event's uuid to
boosted.  For instance, if the event is already in the process of
committing.  However, if that commit goes awry and we backout, we want
the replacement event generated to use a boosted event id, rather than
its original id.  This commit handles that logic.
Each event now has a priority *that is separate from its uuid*.  Using
this approach, we can keep uuids immutable, which means we do not have
to change the maps and lists in active event map, locked object, and
boosted manager.  We still aren't actually changing priorities on
promotion.  Similarly, still must think through strategy for reads and
writes on event priorities so that we do not get conflict/deadlock.