SNMP Alarm Support

I think that in almost all cases where we create a HTTPConnection (or any external connection - new components using the memcachedstore and so on), the right thing to do is to add an alarm for when it fails. Even if there's a case where we don't need one, we should be explicitly considering whether we do. Does that seem reasonable?

Given that, I think it should be a constructor parameter. This means you have to explicitly think, when constructing a new object, about whether it needs alarming - otherwise, someone who is already familiar with HttpConnection or MemcachedStore objects could miss this API change and accidentally omit it rather than having to think about it.

This is consistent with how we've implemented this sort of thing before (such as the stats aggregator and load monitor here, which also represent "optional" features which are not intrinsic to the definition of a HTTPConnection).