Make fUseCrypto atomic

zcash: cherry picked from commit c626dcb
zcash: bitcoin/bitcoin#11107

src/wallet/crypter.h

@@ -13,6 +13,8 @@
 
 class uint256;
 
+#include <atomic>
+
 const unsigned int WALLET_CRYPTO_KEY_SIZE = 32;
 const unsigned int WALLET_CRYPTO_SALT_SIZE = 8;
 const unsigned int WALLET_CRYPTO_IV_SIZE = 16;
@@ -138,7 +140,7 @@ class CCryptoKeyStore : public CBasicKeyStore
 
     //! if fUseCrypto is true, mapKeys, mapSproutSpendingKeys, and mapSaplingSpendingKeys must be empty
     //! if fUseCrypto is false, vMasterKey must be empty
-    bool fUseCrypto;
+    std::atomic<bool> fUseCrypto;
 
     //! keeps track of whether Unlock has run a thorough check before
     bool fDecryptionThoroughlyChecked;