Un-hardcode TX_FEE constants

Allow setting of MIN_TX_FEE / MIN_RELAY_TX_FEE with
-mintxfee / -mintxrelayfee

Default values are the same (0.0001 BTC).

src/qt/bitcoin.cpp

@@ -69,7 +69,7 @@ static bool ThreadSafeAskFee(int64 nFeeRequired)
 {
     if(!guiref)
         return false;
-    if(nFeeRequired < MIN_TX_FEE || nFeeRequired <= nTransactionFee || fDaemon)
+    if(nFeeRequired < CTransaction::nMinTxFee || nFeeRequired <= nTransactionFee || fDaemon)
         return true;
 
     bool payFee = false;