Make an accurate version of 'age' for events
We've always had 'age' in events but it's never really been an
accurate representation of the event's age because we never did
anything with it. This transforms it into a local clock timestamp
when the event arives and when it comes out of the sync store, and
changes getLocalAge() to use it.
react-sdk doesn't appear to use getLocalAge() but any 3rd party apps
that do may notice a slight change in bahaviour.
