This allows the path for the default filter to be specified explicitly
when creating a stream and reduces the need to rely on the default path
function to magically supply the path.
The default path function is now only used if, when a filter is added to
a stream, it has neither a path nor a path function already.
Adapted the existing Log::create_stream calls to explicitly specify a
path value.
Addresses BIT-1324
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.
