wallet: refactor: various master key encryption cleanups

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).