fix(consensus): Refactor block subsidy to handle Height::MAX without panicking

[teor2345]

Motivation

Zebra panics when doing the block subsidy calculations for Height::MAX.

This is unlikely to ever happen on mainnet, because it only happens when block subsidies are all zero zats. But it's good to clean it up anyway, to avoid future changes causing actual panics in production.

Specifications

https://zips.z.cash/protocol/protocol.pdf#subsidies

Designs

Use Option<u64> internally whenever 2^{Halving(height)} would overflow a u64.

Solution

Refactor the halving divisor to avoid overflows and panics.

Add tests for Height::MAX and other large values.

Review

This is a routine consensus rule fix, it would be good to get two people to review it.

This doesn't need to be included in the audit, because it only happens with very large heights.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
• How do you know it works? Does it have tests?

[teor2345]

This isn't needed for anything else now, so it's a low priority.

[oxarbitrage]

I think this looks good so probably better to just merge now instead of leaving it hanging there.