High CPU utilization during transaction spam with only ethrex nodes

[rodrigo-o]

During some devnet spam testing we've seen how ethrex take all cores at 100% when spamoor is active. This takes some time to happen, at least 10 blocks with the current way to reproduce it. This branch provides an easy way to reproduce the issue:

• make localnet-only-ethrex
• Wait for ~10 blocks inspecting the CPU usage with htop
• When all CPUS are at 100% they continue at that level until spamoor is stopped
• make stop-localnet-spamoor
• Wait for 3/4 blocks and look the drop in CPU usage

This could be related to the RPC api or the send/recieve of transactions through p2p between the nodes. We need to dig deeper