scripted-diff: Rename interfaces::WalletClient to interfaces::WalletLoader

Name has been confusing since it was introduced, and it was pointed in
recent review club as https://bitcoincore.reviews/10102 that it was
particularly unclear how interfaces::WalletClient was different from
interfaces::Wallet.

-BEGIN VERIFY SCRIPT-
ren() { git grep -l "$1" src | xargs sed -i "s/$1/$2/g"; }
ren WalletClient WalletLoader
ren walletClient walletLoader
ren wallet_client wallet_loader
ren "wallet clients release the wallet" "wallet pointer owners release the wallet"
ren "wallet client" "wallet loader"
ren "Wallet client" "Wallet loader"
-END VERIFY SCRIPT-

src/qt/walletmodel.cpp

@@ -583,7 +583,7 @@ QString WalletModel::getDisplayName() const
 
 bool WalletModel::isMultiwallet()
 {
-    return m_node.walletClient().getWallets().size() > 1;
+    return m_node.walletLoader().getWallets().size() > 1;
 }
 
 void WalletModel::refresh(bool pk_hash_only)