Transaction: introduce constant for defining dust

Dust is defined by a minimum feerate. Historically this has been based on the
REFERENCE_DEFAULT_MIN_TX_FEE, however changing the dust limit changes which
transactions are standard and should be done with care.

Hence, we decouple the two constants.

Author: schildbach

core/src/main/java/org/bitcoinj/core/Transaction.java

@@ -144,6 +144,11 @@ private static int sortableBlockHeight(Transaction tx) {
      */
     public static final Coin REFERENCE_DEFAULT_MIN_TX_FEE = Coin.valueOf(1_000); // 0.01 mBTC
 
+    /**
+     * Minimum feerate for defining dust.
+     */
+    public static final Coin DUST_RELAY_TX_FEE = Coin.valueOf(3000); // per kB
+
     /**
      * If using this feePerKb, transactions will get confirmed within the next couple of blocks.
      * This should be adjusted from time to time. Last adjustment: February 2017.

core/src/main/java/org/bitcoinj/core/TransactionOutput.java

@@ -241,7 +241,7 @@ else if (ScriptPattern.isP2WH(script))
      * and mined by default miners.
      */
     public Coin getMinNonDustValue() {
-        return getMinNonDustValue(Transaction.REFERENCE_DEFAULT_MIN_TX_FEE.multiply(3));
+        return getMinNonDustValue(Transaction.DUST_RELAY_TX_FEE);
     }
 
     /**