We can only add one block of the given height and we have two competing
goroutines to do that --- consensus and block queue. Whomever adds the block
first shouldn't trigger an error in another one.
Fix block relaying for blocks added via the block queue also, previously one
consensus-generated blocks were broadcasted.
This one will replace blockCache in Blockchain itself as it can and should be
external from it. The idea is that we only feed successive blocks into the
Blockchain and it only stores valid proper Blockchain and nothing else.
