[wallet] Move maxTxFee to wallet

This commit moves the maxtxfee setting to the wallet. There is only
one minor behavior change:

- an error message in feebumper now refers to -maxtxfee instead of
maxTxFee.

src/wallet/feebumper.cpp

@@ -162,9 +162,9 @@ Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoin
     }
 
     // Check that in all cases the new fee doesn't violate maxTxFee
-     const CAmount max_tx_fee = wallet->chain().maxTxFee();
+     const CAmount max_tx_fee = wallet->m_default_max_tx_fee;
      if (new_fee > max_tx_fee) {
-         errors.push_back(strprintf("Specified or calculated fee %s is too high (cannot be higher than maxTxFee %s)",
+         errors.push_back(strprintf("Specified or calculated fee %s is too high (cannot be higher than -maxtxfee %s)",
                                FormatMoney(new_fee), FormatMoney(max_tx_fee)));
          return Result::WALLET_ERROR;
      }