Instead of having a separate CCryptoKeyStore that handles the encryption
stuff, just roll it all into CWallet.
https://github.com/bitcoin/bitcoin/pull/16227/commits/8f5b81e6edae9cb22559545de63f391d97c15701
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Depends on D6558
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16227 | PR16227]]