Fix issues with WebSocket UpgradeRequest after upgrade is complete

[lachlan-roberts]

trafficstars

Issue https://github.com/jetty/jetty.project/issues/11294

• Ensure that JetterServerUpgradeRequest and JetterServerUpgradeResponse are only used for the webscoket negotiation, and are not available from the UpgradeRequest and UpgradeResponse which can be obtained from the Session.
• The UpgradeRequest and UpgradeResponse instances from the Session are now CompletedUpgradeRequest, which will copy any information it needs so it is not accessing the core Request/Response after the upgrade.
• Wrap the servlet request/response as a Core request/response before it is passed to the WebSocketHandshaker.
• Various other cleanups.