refactored menelaus_event detection of interesting events

So that adding new interesting events is easier and less error-prone.

Change-Id: I6ff84b4e592ead697aa814c4f909f4978a2610a2
Reviewed-on: http://review.couchbase.org/35214
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>