B-tree v2.0.0-alpha.68

• Remove debugging message.
• Maybe simplify vacuum.
• Upgrade journalist to 0.1.12.
• Upgrade destructible to 7.0.0-alpha.26.
• Upgrade interrupt to 11.0.0-alpha.14.
• Upgrade turnstile to 6.0.0-alpha.40.
• Upgrade fracture to 0.3.0-alpha.21.
• Use Transcript.
• Replace cache with Magazine.
• Upgrade fracture to 0.3.0-alpha.20.
• Use new Turnstile and Fracture.
• Upgrade journalist to 0.1.10.
• Upgrade reciprocate to 0.0.3.
• Upgrade interrupt to 11.0.0-alpha.11.
• Build with Node.js 12, 14, and 15 on CI/CD.
• Notes and a big TODO.
• Another diary entry on counting and hashes.

Closes #737.