fixed unused() method to properly check sent packets

[Shreyan11]

The old Path::unused() method only looked at whether the path was inactive and missing an active destination connection ID. It didn’t actually check if there were packets still in flight or sitting in the sent queue. That could let a path get marked as unused even though it still had packets hanging around.

This update adds two more checks:

• bytes_in_flight_duration() == Duration::ZERO
• sent_packets_empty()

Now, both LegacyRecovery and GRecovery implement sent_packets_empty(), so they accurately report when their sent packet queues are empyt