SIMD-0207: Raise block limit to 50M

[apfitzge]

Problem

• SIMD-0207: Raise Block Limits to 50M solana-foundation/solana-improvement-documents#207

Summary of Changes

• Add feature to the feature set for raising block limits
• Clean up old limit justification(s)
• Add function to get new limits for SIMD-0207
• Add feature-activation path for SIMD-0207

Fixes #

[apfitzge]

Small patch to raise limits for solana-foundation/solana-improvement-documents#207.

There are 34 lines of test, so "functional" patch is small, and I think should be backported so we can raise chain capacity slightly at a much earlier date.

[tao-stones]

lgtm