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/walletmodel.cpp

@@ -231,12 +231,6 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         return AmountExceedsBalance;
     }
 
-    if((total + nTransactionFee) > nBalance)
-    {
-        transaction.setTransactionFee(nTransactionFee);
-        return SendCoinsReturn(AmountWithFeeExceedsBalance);
-    }
-
     {
         LOCK2(cs_main, wallet->cs_wallet);