Having PingPongDataBroker on the side is troublesome from service injection perspective and it requires multiple brokers to be injected into some applications.

The requisite functionality can easily be supported as a decorator by implementations, hence expand (DOM)TransactionChain with a new createMergingTransactionChain() method, which creates such transaction chain.

Provide PingPongTransactionChain in DOM SPI so implementation's job is easier and use it in the implementations we have.