[neo-project/neo] Staked free tx [idea] (#357)

@PeterLinX we can start to implement filters, and evolve these filters as attacks become «smarter». Right now, we can detect all kinds of known attacks, as they are quite simple. Repeated transactions with similar values (signed by the same staker) will be interpreted as misbehavior. If the attacker don’t have a stake, he will have to pay higher fees, as usual (solution to the general case). The point here is to protect exchanges are legit business on Neo to take fees they don’t deserve.