Merge #13011: Cache witness hash in CTransaction

fac1223a56 Cache witness hash in CTransaction (MarcoFalke)
faab55fbb1 Make CMutableTransaction constructor explicit (MarcoFalke)

Pull request description:

  This speeds up:
  * compactblocks (v2)
  * ATMP
  * validation and miner (via `BlockWitnessMerkleRoot`)
  * sigcache (see also unrelated #13204)
  * rpc and rest (nice, but irrelevant)

  This presumably slows down rescan, which uses a `CTransaction` and its `GetHash`, but never uses the `GetWitnessHash`. The slow down is proportional to the number of witness transactions in the rescan window. I.e. early in the chain there should be no measurable slow down. Later in the chain, there should be a slow down, but acceptable given the speedups in the modules mentioned above.

Tree-SHA512: 443e86acfcceb5af2163e68840c581d44159af3fd1fce266cab3504b29fcd74c50812b69a00d41582e7e1c5ea292f420ce5e892cdfab691da9c24ed1c44536c7

src/bitcoin-tx.cpp

@@ -546,7 +546,7 @@ static void MutateTxSign(CMutableTransaction& tx, const std::string& flagStr)
     // mergedTx will end up with all the signatures; it
     // starts as a clone of the raw tx:
     CMutableTransaction mergedTx{tx};
-    const CTransaction txv{tx};
+    const CMutableTransaction txv{tx};
     CCoinsView viewDummy;
     CCoinsViewCache view(&viewDummy);