change(consensus): Add lockbox funding stream

[arya2]

Motivation

We likely want to add a lockbox funding stream.

Will close #8686, and close #8687 once parameters/addresses are updated.

Part of #8695.

This PR will need to be updated once there's more information about which dev fund proposal we should implement, and, if it's alternative 2 in this draft ZIP, also what funding stream addresses to use for ZCG.

Specifications & References

Alternatives 2 and 3 in https://zips.z.cash/draft-nuttycom-funding-allocation
https://zips.z.cash/draft-nuttycom-lockbox-streams

Solution

• Adds a new funding stream receiver for the "deferred pool"
• Checks funding streams after the next halving
• Generates coinbase transactions with new funding streams after NU6 is active
• Checks that block subsidies are fully claimed after NU6 is active
• Checks that coinbase transactions spend less than:
  block_subsidy + miner_fees - expected_lockbox_funding_stream_amount
• Adds a lockbox_input_value() function

Related changes:

• Factors Halving(height) calculation into its own num_halvings() function
• In block::check::subsidy_is_valid():
  • Removes unnecessary check that halving_div.count_ones() == 1
  • Removes redundant check for height > slow_start_shift
  • Checks num_halvings() instead of halving_divisor()
  • Minor simplifications

Tests

• Adds an acceptance test that relies on configurable funding streams on Testnet
• Updates test to check NU6 funding stream values
• Adds a check_lockbox_input_value() test for the lockbox_input_value() fn

Follow Up Work

• change: Track the balance of the deferred chain value pool #8729
• change(consensus): Require that coinbase transactions balance exactly after NU6 activation #8727

Blocked on ZIP-253 status being changed to 'Proposed':

• Set an NU6 activation height for Testnet
• Update minimum network protocol versions for NU6 on Testnet/Mainnet
  
• Update initial current minimum network protocol version
• Update post-NU6 funding stream height ranges
• Update the NU6 consensus branch id
• Update the end of support halt time

This PR also adds some TODOs for documentation updates that should be addressed promptly.

PR Author's Checklist

• The PR name will make sense to users.
• The PR provides a CHANGELOG summary.
• The solution is tested.
• The documentation is up to date.
• The PR has a priority label.

PR Reviewer's Checklist

• The PR Author's checklist is complete.
• The PR resolves the issue.

[oxarbitrage]

Left some comments and questions.

[PaulLaux]

added some minor comments

[conradoplg]

Looks good, I unresolved one conversation since I think you might have missing one change that you wanted to do, but I don't think it's required, so feel free to address it or mark as resolved