core: Added new TD strategy which mitigate the risk for selfish mining

Assuming the following scenario where a miner has 15% of all hashing
power and the ability to exert a moderate control over the network to
the point where if the attacker sees a message A, it can't stop A from
propagating, but what it **can** do is send a message B and ensure that
most nodes see B before A. The attacker can then selfish mine and
augment selfish mining strategy by giving his own blocks an advantage.

This change makes the time at which a block is received less relevant
and so the level of control an attacker has over the network no longer
makes a difference.

This change changes the current td algorithm `B_td > C_td` to the new
algorithm `B_td > C_td || B_td == C_td && rnd < 0.5`.

tests/init.go

@@ -44,6 +44,9 @@ var (
 		"TRANSCT__RandomByteAtTheEnd",
 		"BLOCK__ZeroByteAtTheEnd",
 		"TRANSCT__ZeroByteAtTheEnd",
+
+		"ChainAtoChainB_blockorder2",
+		"ChainAtoChainB_blockorder1",
 	}
 
 	/* Go client does not support transaction (account) nonces above 2^64. This