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/dummywallet.cpp

@@ -12,7 +12,7 @@ namespace interfaces {
 class Chain;
 class Handler;
 class Wallet;
-class WalletClient;
+class WalletLoader;
 }
 
 class DummyWalletInit : public WalletInitInterface {
@@ -64,7 +64,7 @@ std::unique_ptr<Wallet> MakeWallet(const std::shared_ptr<CWallet>& wallet)
     throw std::logic_error("Wallet function called in non-wallet build.");
 }
 
-std::unique_ptr<WalletClient> MakeWalletClient(Chain& chain, ArgsManager& args)
+std::unique_ptr<WalletLoader> MakeWalletLoader(Chain& chain, ArgsManager& args)
 {
     throw std::logic_error("Wallet function called in non-wallet build.");
 }