Merge #19453: refactor: reduce DefaultRequestHandler memory allocations

f20b359 cli: reduce DefaultRequestHandler memory allocations (Jon Atack)

Pull request description:

  per bitcoin/bitcoin#16439 (comment). Simpler code, fewer allocations. No change of behavior. The code has good test coverage in `interface_bitcoin_cli.py`.

ACKs for top commit:
  MarcoFalke:
    review ACK f20b359
  fjahr:
    Code review ACK f20b359

Tree-SHA512: 745eab44dfdcc485ca2bbc1db8b8d364cbd3cf94982e46e033745ce05ab617c15320ee55da7fb930d365f4d26b172049d5f5efcf0b6d3af5b0a28185bdb93ea8

src/bitcoin-cli.cpp

@@ -516,16 +516,16 @@ static void ParseError(const UniValue& error, std::string& strPrint, int& nRet)
  */
 static void GetWalletBalances(UniValue& result)
 {
-    std::unique_ptr<BaseRequestHandler> rh{MakeUnique<DefaultRequestHandler>()};
-    const UniValue listwallets = ConnectAndCallRPC(rh.get(), "listwallets", /* args=*/{});
+    DefaultRequestHandler rh;
+    const UniValue listwallets = ConnectAndCallRPC(&rh, "listwallets", /* args=*/{});
     if (!find_value(listwallets, "error").isNull()) return;
     const UniValue& wallets = find_value(listwallets, "result");
     if (wallets.size() <= 1) return;
 
     UniValue balances(UniValue::VOBJ);
     for (const UniValue& wallet : wallets.getValues()) {
         const std::string wallet_name = wallet.get_str();
-        const UniValue getbalances = ConnectAndCallRPC(rh.get(), "getbalances", /* args=*/{}, wallet_name);
+        const UniValue getbalances = ConnectAndCallRPC(&rh, "getbalances", /* args=*/{}, wallet_name);
         const UniValue& balance = find_value(getbalances, "result")["mine"]["trusted"];
         balances.pushKV(wallet_name, balance);
     }
@@ -540,8 +540,8 @@ static UniValue GetNewAddress()
 {
     Optional<std::string> wallet_name{};
     if (gArgs.IsArgSet("-rpcwallet")) wallet_name = gArgs.GetArg("-rpcwallet", "");
-    std::unique_ptr<BaseRequestHandler> rh{MakeUnique<DefaultRequestHandler>()};
-    return ConnectAndCallRPC(rh.get(), "getnewaddress", /* args=*/{}, wallet_name);
+    DefaultRequestHandler rh;
+    return ConnectAndCallRPC(&rh, "getnewaddress", /* args=*/{}, wallet_name);
 }
 
 /**