reset pending keepalive state on listener handoff during reconfiguration

[ZECHEESELORD]

Fixes PaperMC/Velocity#1723. (The backend side)

During mid session reconfiguration behind Velocity, Paper was carrying the same KeepAlive tracker across listener handoff. That allowed a pending PLAY keepalive expectation to survive into the CONFIG listener.

In the failing case, the sequence was:

1. the PLAY listener issued keepalive A
2. listener handoff moved the player into CONFIG
3. the CONFIG listener inherited the pending expectation for A
4. the CONFIG listener issued keepalive B
5. the reply for B arrived while the backend still considered A outstanding
6. Paper rejected B as stale or out-of-order and disconnected the player with disconnect.timeout

The key signal from backend logging was that listener_handoff showed the CONFIG listener inheriting the prior PLAY expectedId, and the next CONFIG keepalive reply was then rejected as stale_or_out_of_order.

So the failure was not that Velocity dropped the CONFIG reply (the backend received it). The problem was that stale pending PLAY keepalive state leaked across the PLAY -> CONFIG handoff.

the fix:

ServerCommonPacketListenerImpl#createCookie(...) now gives the next listener a fresh KeepAlive instance instead of reusing the current listener's pending tracker. This preserves latency/history state, but intentionally resets inflight keepalive challenge state across listener handoff. In other words:

• preserve ping history
• reset pending keepalive expectations

That restores the expected protocol boundary between PLAY and CONFIG.

testing:

Tested locally with the Velocity reproduction from PaperMC/Velocity#1723.

Before this fix:

• repeated config/unconfig cycles would eventually fail
• failures were much easier to trigger with added latency
• backend logs showed CONFIG replies being rejected as out-of-order because the old PLAY expectation was still present

After this fix:

• no disconnects at 0 ms added latency
• no disconnects at 200 ms added latency
• no disconnects at 800 ms added latency
• backend logs show the CONFIG listener no longer inherits the stale PLAY pending keepalive expectation