Allow the long term feerate to be configured, default of 10 sat/vb

The long term feerate is really the highest feerate that the user is
comfortable with making consolidatory transactions. This is should thus
be something that can be configured by the user via a new startup option
-consolidatefeerate. The default value is 10 sat/vbyte, chosen
arbitrarily (it seems like a reasonable number).

src/wallet/wallet.cpp

@@ -2701,6 +2701,15 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
         walletInstance->m_default_max_tx_fee = max_fee.value();
     }
 
+    if (gArgs.IsArgSet("-consolidatefeerate")) {
+        if (std::optional<CAmount> consolidate_feerate = ParseMoney(gArgs.GetArg("-consolidatefeerate", ""))) {
+            walletInstance->m_consolidate_feerate = CFeeRate(*consolidate_feerate);
+        } else {
+            error = AmountErrMsg("consolidatefeerate", gArgs.GetArg("-consolidatefeerate", ""));
+            return nullptr;
+        }
+    }
+
     if (chain && chain->relayMinFee().GetFeePerK() > HIGH_TX_FEE_PER_KB) {
         warnings.push_back(AmountHighWarn("-minrelaytxfee") + Untranslated(" ") +
                            _("The wallet will avoid paying less than the minimum relay fee."));