Type-safe CFeeRate class

Use CFeeRate instead of an int64_t for quantities that are
fee-per-size.

Helps prevent unit-conversion mismatches between the wallet,
relaying, and mining code.

src/qt/paymentserver.cpp

@@ -551,7 +551,7 @@ bool PaymentServer::processPaymentRequest(PaymentRequestPlus& request, SendCoins
 
         // Extract and check amounts
         CTxOut txOut(sendingTo.second, sendingTo.first);
-        if (txOut.IsDust(CTransaction::nMinRelayTxFee)) {
+        if (txOut.IsDust(CTransaction::minRelayTxFee)) {
             emit message(tr("Payment request error"), tr("Requested payment amount of %1 is too small (considered dust).")
                 .arg(BitcoinUnits::formatWithUnit(optionsModel->getDisplayUnit(), sendingTo.second)),
                 CClientUIInterface::MSG_ERROR);