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/rpcwallet.cpp

@@ -1883,7 +1883,7 @@ Value settxfee(const Array& params, bool fHelp)
     if (params[0].get_real() != 0.0)
         nAmount = AmountFromValue(params[0]);        // rejects 0.0 amounts
 
-    nTransactionFee = nAmount;
+    payTxFee = CFeeRate(nAmount, 1000);
     return true;
 }