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/optionsdialog.cpp

@@ -14,7 +14,7 @@
 #include "monitoreddatamapper.h"
 #include "optionsmodel.h"
 
-#include "main.h" // for CTransaction::nMinTxFee and MAX_SCRIPTCHECK_THREADS
+#include "main.h" // for CTransaction::minTxFee and MAX_SCRIPTCHECK_THREADS
 #include "netbase.h"
 #include "txdb.h" // for -dbcache defaults
 
@@ -101,7 +101,7 @@ OptionsDialog::OptionsDialog(QWidget *parent) :
 #endif
 
     ui->unit->setModel(new BitcoinUnits(this));
-    ui->transactionFee->setSingleStep(CTransaction::nMinTxFee);
+    ui->transactionFee->setSingleStep(CTransaction::minTxFee.GetFeePerK());
 
     /* Widget-to-option mapper */
     mapper = new MonitoredDataMapper(this);