wallet: refactor: various master key encryption cleanups #31398

1. 
   theStack commented at 3:54 am on December 1, 2024: contributor

   This PR contains various cleanups around the wallet’s master key encryption logic. The default/minimum key derivation rounds magic number of 25000 is hoisted into a constant (member of CMasterKey) and two new functions EncryptMasterKey/DecryptMasterKey are introduced in order to deduplicate code for the derivation round determination and master key en/decryption. Also, mentions of the never-implemented derivation method scrypt are removed from the wallet crypter header and both plain and encrypted master key instances are renamed to adapt to moderning coding style (hopefully improving readability).
2. wallet: doc: remove mentions of unavailable scrypt derivation method

   These comments are there since wallet encryption was first introduced
   (see commit 4e87d341f75f13bbd7d108c31c03886fbc4df56f, PR #352), but scrypt
   was actually never implemented as a derivation method.

   a6af5b45a1
3. wallet: refactor: introduce `CMasterKey::DEFAULT_DERIVE_ITERATIONS` constant

   This gets rid of the magic number used in both the `CMasterKey` ctor
   and the wallet encryption / passphrase change methods.

   22e9869524
4. wallet: refactor: dedup master key encryption / derivation rounds setting de9b54780b
5. wallet: refactor: dedup master key decryption ba9f582332
6. scripted-diff: wallet: rename plain and encrypted master key variables

   -BEGIN VERIFY SCRIPT-
   sed -i s/_vMasterKey/plain_master_key/g ./src/wallet/wallet.cpp
   sed -i s/kMasterKey/master_key/g ./src/wallet/wallet.cpp
   sed -i "s/const MasterKeyMap::value_type& pMasterKey/const auto\& \[_, master_key\]/g" ./src/wallet/wallet.cpp
   sed -i s/pMasterKey\.second/master_key/g ./src/wallet/wallet.cpp
   sed -i "s/MasterKeyMap::value_type& pMasterKey/auto\& \[master_key_id, master_key\]/g" ./src/wallet/wallet.cpp
   sed -i s/pMasterKey\.first/master_key_id/g ./src/wallet/wallet.cpp
   -END VERIFY SCRIPT-

   bda3a48190
7. 
   DrahtBot commented at 3:54 am on December 1, 2024: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31398.

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8. DrahtBot added the label Wallet on Dec 1, 2024
9. 
   furszy commented at 6:04 pm on December 27, 2024: member

   Concept ACK, nice code dedup.

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Wallet