Merge bitcoin/bitcoin#22675: RBF move 2/3: extract RBF logic into policy/rbf

32748da0f47f7aa9fba78dfb29aa426b14f15624 whitespace fixups after move and scripted-diff (glozow)
fa47622e8dc66bec9ea690aec3f0999108d76dc9 scripted-diff: rename variables in policy/rbf (glozow)
ac761f0a23c9c469fa00885edf3d5c9ae7c6a2b3 MOVEONLY: fee checks (Rules 3 and 4) to policy/rbf (glozow)
9c2f9f89846264b503d5573341bb78cf609cbc5e MOVEONLY: check that fees > direct conflicts to policy/rbf (glozow)
3f033f01a6b0f7772ae1b21044903b8f4249ad08 MOVEONLY: check for disjoint conflicts and ancestors to policy/rbf (glozow)
7b60c02b7d5e2ab12288393d2258873ebb26d811 MOVEONLY: BIP125 Rule 2 to policy/rbf (glozow)
f8ad2a57c61d1e817e2445226688e03080fc8688 Make GetEntriesForConflicts return std::optional (glozow)

Pull request description:

  This PR does not change behavior. It extracts the BIP125 logic into helper functions (and puts them in the policy/rbf* files). This enables three things - I think each one individually is pretty good:

  - Implementation of package RBF (see #22290). I want it to be as close to BIP125 as possible so that it doesn't become a distinct fee-bumping mechanism. Doing these move-only commits first means the diff is mostly mechanical to review, and I just need to create a function that mirrors the single transaction validation.
  - We will be able to isolate and test our RBF logic alone. Recently, there have been some discussions on discrepancies between our code and BIP125, as well as proposals for improving it. Generally, I think making this code more modular and de-bloating validation.cpp is probably a good idea.
  - Witness Replacement (replacing same-txid-different-wtxid when the witness is significantly smaller and therefore higher feerate) in a BIP125-similar way. Hopefully it can just be implemented with calls to the rbf functions (i.e. `PaysForRBF`) and an edit to the relevant mempool entries.

ACKs for top commit:
  mjdietzx:
    ACK 32748da0f47f7aa9fba78dfb29aa426b14f15624
  theStack:
    Code-review ACK 32748da0f47f7aa9fba78dfb29aa426b14f15624 📐
  MarcoFalke:
    review ACK 32748da0f47f7aa9fba78dfb29aa426b14f15624 🦇

Tree-SHA512: d89985c8b4b42b54861018deb89468e04968c85a3fb1113bbcb2eb2609577bc4fd9bf254593b5bd0e7ab059a0fa8192d1a903b00f77e6f120c7a80488ffcbfc0

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