SNMP Alarm Support

Based on http://www.developingsolutions.com/solutions/diameter_result_codes/, 4002, 4003, 5001,5002, 5004, 5005, 5007, 5008, 5009, 5010, 5011, 5013, 5014, 5015 and 5017 look like likely ones from the base protocol, From 3GPP-defined codes, 5011 and 4181 are the only ones that look relevant.

This might be something we want to keep an eye on (as in, "what sequence of Diameter errors triggered this alarm?") - I've suggested some more logging in https://github.com/Metaswitch/cpp-common/pull/185/files#r18222281 to help track any problems down.