Reduce bad txs with wrong sequence numbers sent by spammers

[philipstanislaus]

Spammers currently produce 5-15% bad txs with wrong sequence numbers. The reason is probably that sometimes one spammer is sending 2 txs so close to each other that the second tx with sequence n+1 arrives before the first with sequnce n. We can reduce the amount of bad txs by enforcing a minimum wait time between two txs from the same spammer, e. g. 2x latency.