Replace CENT with new constant MIN_TX_FEE, where appropriate.

MIN_TX_FEE==CENT remains true (until next commit).

src/main.cpp

@@ -741,7 +741,7 @@ bool CTransaction::AcceptToMemoryPool(CTxDB& txdb, bool fCheckInputs, bool* pfMi
         // Continuously rate-limit free transactions
         // This mitigates 'penny-flooding' -- sending thousands of free transactions just to
         // be annoying or make other's transactions take longer to confirm.
-        if (nFees < CENT)
+        if (nFees < MIN_TX_FEE)
         {
             static CCriticalSection cs;
             static double dFreeCount;