Load balancer backends

In proxyd a backend is considered healthy if it meets the following criteria

• Not banned
• Avg 1-min moving window error rate ≤ configurable threshold
• Avg 1-min moving window latency ≤ configurable threshold
• Peer count ≥ configurable threshold
• Latest block lag ≤ configurable threshold
• ast state update ≤ configurable threshold
• Not currently syncing