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

@PeterLinX if you all agree, we can try to quickly propose some simple implementation of staking, without any punishment at all, just to allow exchanges/business to escape general price changes. Obviously, the bad actors could still attack since they know there’s no filtering… but I doubt someone will risk exposing itself with a high stake just to try some random attacks. I believe attacks will cease even without any filters.