Small refactroing of transaction_pool.ml

[georgeee]

Prettify code a little bit, move some code around.

This refactoring is useful for a much large effort in #16556.

Explain how you tested your changes:

• An extra-simple refactoring, and it compiles

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? None

[georgeee]

!ci-build-me