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. 

AFAIS, the code seems to do what you say. But I don't see your point on why it works.

The wallet and the actor observer are different listeners. Each listener is asked separately for the relevance of the tx. So the fact that the wallet accepts it shouldn't have any effect on the actor observer.