Adds a checksum to the encrypted key record in the wallet database so that encrypted keys can be checked for corruption on wallet loading, in the same way that unencrypted keys are. This allows for us to skip the full decryption of keys upon the first unlocking of the wallet in that session as any key corruption will have already been detected. The checksum is just the double SHA256 of the encrypted key and it is appended to the record after the encrypted key itself.

This is backwards compatible as old wallets will be able to read the encrypted key and ignore that there is more data in the stream. Additionally, old wallets will be upgraded upon their first unlocking (so that key decryption is checked before we commit to a checksum of the encrypted key) and a wallet flag set indicating that. The presence of the wallet flag lets us skip the full decryption as if `fDecryptionThoroughlyChecked` were true.

This does mean that the first time an old wallet is unlocked in a new version will take much longer, but subsequent unlocks will be instantaneous. Furthermore, corruption will be detected upon loading rather than on trying to send so wallet corruption will be detected sooner.

Fixes #12423

ninja all check check-functional