Introduce fail rate

### Summary
- Develop a mechanism that prevents nodes from responding to received requests, ie. nodes receive messages/requests but fail to forward/respond to them.
- Nodes that fail - stay online.
- Feel free to make modifications to the `discv5` code

### Settings
- Moderate: **5%** of nodes are unresponsive
- High: **10%** of nodes are unresponsive

### Notes
- This is worse than pure churn because, as it prevents the sender from looking for alternative routes unless ACKs and resubmissions are applied.