SNMP Alarms Release-58

I'm happy with this approach (and it's basically analogous to the "memcached vBucket is inaccessible" alarm). Generally, I'm happy with the case where N things are down and that's a definitive alarm condition (e.g. losing two memcacheds or two Cassandrai makes data inaccessible).

What I think I was sceptical about is an approach where we pick an arbitrary number to alarm on (e.g. "my last 20 requests to Cassandra have failed, so raise an alarm saying that everything is down"), because that feels like something we'll just have to tune and retune to make it work for all deployment sizes and shapes.