[neo-project/neo] Network congestion attacks (#353)

Very much appreciate the quick responses guys, looking forward to the merging of the above code fixes so you and the community can all move on with life without these malicious attacks. The fact that parties are conducting these attacks is encouraging I guess from two aspects

1. Gives you a chance to make the network more robust
2. Means parties out there feel threatened by NEO

All positive things

Thanks again