Without this change, flow labeling of connections over IPv6 are
only available in the per-packet types of events (e.g. new_packet)
in which header fields can be inspected, but now minimal tracking
of the most recent flow label is done internally and that's available
per-connection for all events that use connection record arguments.
Specifically, this adds a "flow_label" field to the "endpoint" record
type, which is used for both the "orig" and "resp" fields of
"connection" records. The new "connection_flow_label_changed" event
also allows tracking of changes in flow labels: it's raised each time
one direction of the connection starts using a different label.
