minor tidies for cassandra
suppress membership-change notifications on unchanged service-up events (removes quite a bit of unnecessary processing and logging) and toString for better logging
tidies for api convenience around enrichers and sensor/feeds, and better logging (esp around location problems)
allow a grace period for sensors logging at WARN level if we haven't received any successful responses yet
Merge pull request #821 from ahgittin/fix/misc3

api conveniences, cassandra tidies, and logging fixes