Migrate the IPUDPBenchmark to use `BridgeDatagramProtocol` instead of shuffling packets with loopOutputHandlerPackets

In https://github.com/apple/swift-network-evolution/pull/16 I updated QUICTransfer to use the more modern technique of using `BridgeDatagramProtocol` to transfer packets at the bottom instead of using the utility function `loopOutputHandlerPackets`. This issue is to also update the IPUDPBenchmark to use `BridgeDatagramProtocol` as well.

Start by reviewing the QUICTransfer changes in pull request 16 to understand how BridgeDatagramProtocol replaces loopOutputHandlerPackets, then locate IPUDPBenchmark. Update that benchmark to use the same packet-transfer approach and verify that it no longer relies on loopOutputHandlerPackets.

Written by the indexing model from the issue text.