Deprecate Event.createdAt() in favor of a version that returns an Instant

Event.createdAt() returns the value of System.nanoTime() captured when the event was created. This great if you want to answer the question, "How many nanoseconds have elapsed since this event occurred?" but makes it inconvenient to get the event's timestamp.

Modifications:

Add a new Event.created() method that returns an Instant representing the event's timestamp.

Deprecate Event.createdAt(), since nanoTime is confusing for everyone, and sub-millisecond resolution is not critical for the event timestamps.