Implement `DefaultBlockchainActor`

Provide the default implementation of blockchain actor. After a refactor of the messages of `BlockchainActor`, the default implementation and its unit test are coded. 

Not yet. After reading the Bitcoinj code I'm not sure that's a bug in their side. Perhaps it was caused by the java-to-scala collection converters (I doubt it, but that's possible). But the experience is that the list has an arbitrary order indeed.