Merge ElementsProject#660: FundTransaction: mark all change keys as kept

dd757ad FundTransaction: mark all change keys as kept (Gregory Sanders)

Pull request description:

  Regression from 0.14 due to internal API changes.

  If a key has already been marked returned it's a nop.

Tree-SHA512: c3842f17a9f6032500491c303bf0bc47b05d1f1fb7b6f37b55fc0815d7b20b30be97e34793526288dccee449e3939df8d6dc79899c1adcb3cf6abf8713700dee

src/wallet/wallet.cpp

@@ -2866,6 +2866,11 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
         }
     }
 
+    // Mark all un-returned change keys as used to reduce privacy loss
+    for (auto& changekey : vChangeKey) {
+        changekey->KeepKey();
+    }
+
     return true;
 }