WIP POC FEATURE: Content stream fork buffer

[mhsdesign]

depends on #5735

see #5718 (comment)
see #5264

Upgrade instructions

Review instructions

Checklist

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the correct branch:
  • If it's a bugfix, use the lowest maintained branch which has the bug
  • If it's a non-breaking feature, use the branch of the next version (might be either minor or major)
  • If it's a breaking feature it should typically go into the next major version
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions