Implementation of block_response is abstract but has to be called by the developer itself

[devos50]

I found that the implementation of the block_response is required. Therefore, I would expect that this implementation is critical when running a simple community derived from the backbone. However, the PaymentCommunity reveals that the developer has to call the block_response manually when processing incoming blocks. I think it would be helpful to move the primitives for confirming/responding to blocks to the backbone. For example, we can provide a method in the backbone, should_counter_sign, that is called on every incoming block, and return True when we respond to that block. Then, the block_response method is called.