Add SNMP datagram parsing support.

This supports parsing of SNMPv1 (RFC 1157), SNMPv2 (RFC 1901/3416), and
SNMPv2 (RFC 3412).  An event is raised for each SNMP PDU type, though
there's not currently any event handlers for them and not a default
snmp.log either.  However, simple presence of SNMP is currently visible
now in conn.log service field and known_services.log.
Fix compiler nitpicks from new SNMP code.
Add memory leak unit test for SNMP.
Merge remote-tracking branch 'origin/topic/jsiwek/snmp'

* origin/topic/jsiwek/snmp:
  Add memory leak unit test for SNMP.
  Fix compiler nitpicks from new SNMP code.
  Add SNMP datagram parsing support.

BIT-1142
Updating test baselines.