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/guiutil.cpp

@@ -210,7 +210,7 @@ bool isDust(const QString& address, qint64 amount)
     CTxDestination dest = CBitcoinAddress(address.toStdString()).Get();
     CScript script; script.SetDestination(dest);
     CTxOut txOut(amount, script);
-    return txOut.IsDust(CTransaction::nMinRelayTxFee);
+    return txOut.IsDust(CTransaction::minRelayTxFee);
 }
 
 QString HtmlEscape(const QString& str, bool fMultiLine)