[neo-project/neo] Proposal for Mempool Congestion (#358)

**Problem 1**

Like this a lot. We had some spam a while back where single transactions had huge sizes. If we implement a minimum transaction fee, I’d like to see it scale based on transaction size.

**Problem 2**

How would the proposed _minRelayFee_ affect services that use many small transactions, e.g Effect.ai?

**Problem 3**

I like the idea of a TTL for transactions in the mempool, but I think it needs to be fine-tuned to ensure the average user experience is not harmed, even during spam. I think TTL should scale based on the number of transactions that address has in the mempool. People are going to get very frustrated if they send one transaction that goes nowhere for 5-10 minutes and find out it has been dropped.