Unbounded growth of pending connection ID retirements in quinn-proto

Summary

When processing a NEW_CONNECTION_ID frame, quinn-proto queues a RETIRE_CONNECTION_ID frame for each connection ID that the peer asked it to retire. The normal path bounds this queue and closes the connection with a CONNECTION_ID_LIMIT_ERROR if the bound would be exceeded. However, the path handling a connection ID that had already been retired pushed onto the queue without applying the limit and without deduplicating.

A peer that has completed the handshake can first send a NEW_CONNECTION_ID frame with a large `retire_prior_to` value to advance the receiver's connection ID window, and then repeatedly send NEW_CONNECTION_ID frames with sequence numbers below that window. Each such frame adds another entry to the queue. The queue is only drained by sending RETIRE_CONNECTION_ID frames, which are subject to congestion control, so a peer that withholds acknowledgements can stall the drain while continuing to flood, growing the victim's memory use without bound.

The flaw was corrected in 0.11.17 by applying the same limit on the already-retired path.

Thanks to [@scadastrangelove](https://github.com/scadastrangelove) for reporting this issue.