Tracking: Implement NU6 ZIPs

[mpguerra]

Motivation

Once the full list of NU6 ZIPs is defined, we should implement them in Zebra. This issue will track implementation of each zip, e.g

• [ ] #8695
• [x] #8688 for "Lockbox Funding Streams" ZIP
• [x] Implement "Block Reward Allocation for Non-Direct Development Funding" ZIP
• [x] #8696 "Blocks should balance exactly" ZIP

[conradoplg]

I added "Blocks should balance exactly"