feat(quinn-proto): Use RTT from path challenges, implement path challenge resending

[matheus23]

• Adds a timer to resend another path challenge if the old one was lost and the path isn't validated.
• The resent challenges now have different tokens. We keep track of all of them to verify incoming path responses.
• The logic for when to send a path challenge is simplified to only send challenges when send_new_challenge is set to true (instead of triggering always when we're in path validation mode)