Integrate TransactionChain transaction merging into the API contract

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.