Merge bitcoin/bitcoin#24410: [kernel 2a/n] Split hashing/index GetUTXOStats codepaths, decouple from coinstatsindex

664a14ba7c coinstats: Move GetUTXOStats to rpc/blockchain (Carl Dong)
f100687566 kernel: Use ComputeUTXOStats in validation (Carl Dong)
faa52387e8 style-only: Rearrange using decls after scripted-diff (Carl Dong)
f329a9298c scripted-diff: Move src/kernel/coinstats to kernel:: (Carl Dong)
0e54456f04 Use only kernel/coinstats.h in index/coinstatsindex.h (Carl Dong)
80970985c9 coinstats: Split node/coinstats.h to kernel/coinstats.h (Carl Dong)
35f73ce4b2 coinstats: Move hasher codepath to kernel/coinstats (Carl Dong)
b7634fe02b Move logic from LookupUTXOStatsWithIndex to CoinStatsIndex::LookUpStats (Carl Dong)
1352e410a5 coinstats: Separate hasher/index lookup codepaths (Carl Dong)
524463daf6 coinstats: Return purely out-param CCoinsStats (Carl Dong)
46eb9fc56a coinstats: Extract index_requested in-member to in-param (Carl Dong)
a789f3f2b8 coinstats: Extract hash_type in-member to in-param (Carl Dong)
102294898d includes: Remove rpc/util.h -> node/coinstats.h (Carl Dong)
0848db9c35 fuzz: Remove useless GetUTXOStats fuzz case (Carl Dong)
52b1939993 kernel: Remove unnecessary blockfilter{index,}.cpp (Carl Dong)

Pull request description:

  Part of: #24303
  Depends on: #24322

  The `GetUTXOStats` function has 2 codepaths:
    - One which queries the `CoinStatsIndex` for the UTXO hash
    - One which actually performs the hashing

  For `libbitcoinkernel`, the only place where we call `GetUTXOStats` is in `PopulateAndValidateSnapshots`, which uses the `SHA256D` hash, and is therefore unable to use the `CoinStatsIndex` since that only provides `MuHash` hashes. Not that I think indices necessarily belong in `libbitcoinkernel` anyway.

  This PR separates these 2 aforementioned codepaths of `GetUTXOStats`, uses the hashing codepath in `PopulateAndValidateSnapshots`, and removes the need to link in `index/coinstatsindex.cpp` and `node/coinstats.cpp`.

  -----

  Logistically, this PR:
  - Extracts out the `index_requested` and `hash_type` members of `CoinStats`, which served as "in-params" to `GetUTXOStats` embedded within the `CoinStats` struct. This allows `CoinStats` to only consist of "out-param" members, and be returned by `GetUTXOStats` without needing to be an "in-out" param
  - Introduce the purely virtual `UTXOHashers` class, with 3 implementations: `SHA256DHasher`, `MuHashHasher`, and `NullHasher`. These replace the existing template-based polymorphism.
  - Split `GetUTXOStats` into:
      - `CalculateUTXOStatsWithHasher(UTXOHasher&, ...)`, and
      - `LookupUTXOStatsWithIndex(CoinStatsIndex&, ...)`
  - Use `CalculateUTXOStatsWithHasher` directly where appropriate (`src/validation.cpp` and `src/fuzz`)
  - Move `GetUTXOStats` to `rpc/blockchain`, which is the only place that depends on `GetUTXOStats`'s weird fallback behaviour
  - Move `LookupUTXOStatsWithIndex` to `index/coinstatsindex`

  Code organization:
  - `src/`
    - `kernel/` → only contains the hashing codepath
      - `coinstats.cpp` → hashing codepath implementations
      - `coinstats.h` → header for `kernel/coinstats.cpp`
    - `index/` → only contains the index codepath
      - `coinstatsindex.cpp` → index codepath implementations
      - `coinstatsindex.h`
    - `validation.cpp` → only uses the hashing codepath
    - `rpc/blockchain.cpp` → uses both the hashing and index codepath, old `GetUTXOStats` fallback logic moved here as static
    - `test/fuzz/coins_view.cpp` → only uses the hashing codepath

  TODOs:
  - [x] Commit messages could be fleshed out more

  Would love any feedback!

ACKs for top commit:
  laanwj:
    Code review ACK 664a14ba7c

Tree-SHA512: 18722c7bd279174d2d1881fec33ea04a9b261aae1c12e998cf434ef297d8ded47de69c526c8033a2ba7abc93ba3d2ff5faf4ce05e8888c725c31cf885ce3ef73

test/lint/lint-circular-dependencies.py

@@ -14,7 +14,6 @@ import sys
 EXPECTED_CIRCULAR_DEPENDENCIES = (
     "chainparamsbase -> util/system -> chainparamsbase",
     "node/blockstorage -> validation -> node/blockstorage",
-    "index/coinstatsindex -> node/coinstats -> index/coinstatsindex",
     "policy/fees -> txmempool -> policy/fees",
     "qt/addresstablemodel -> qt/walletmodel -> qt/addresstablemodel",
     "qt/recentrequeststablemodel -> qt/walletmodel -> qt/recentrequeststablemodel",
@@ -22,7 +21,7 @@ EXPECTED_CIRCULAR_DEPENDENCIES = (
     "qt/transactiontablemodel -> qt/walletmodel -> qt/transactiontablemodel",
     "wallet/fees -> wallet/wallet -> wallet/fees",
     "wallet/wallet -> wallet/walletdb -> wallet/wallet",
-    "node/coinstats -> validation -> node/coinstats",
+    "kernel/coinstats -> validation -> kernel/coinstats",
 )
 
 CODE_DIR = "src"