CircularJSON is now deprecated, so replace it with its successor
and use it in the sole location where we used CircularJSON, ie
in the LoggingEvent serialization/deserialization functions
* npm - install flatted, remove circular-json
* LoggingEvent - use flatted for serialization/deserialization
* tests - acknowledge that flatted (and circular-json) should not
parse anything other than their respective serialized payloads
and use flatted when parsing/stringifying test data
