Add Wallet.reset() to prepare the wallet for a blockchain replay.

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

@@ -2474,6 +2474,25 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
         }
     }
 
+    /**
+     * Prepares the wallet for a blockchain replay. Removes all transactions (as they would get in the way of the
+     * replay) and makes the wallet think it has never seen a block. {@link WalletEventListener#onWalletChanged()} will
+     * be fired.
+     */
+    public void reset() {
+        lock.lock();
+        try {
+            clearTransactions();
+            lastBlockSeenHash = null;
+            lastBlockSeenHeight = -1; // Magic value for 'never'.
+            lastBlockSeenTimeSecs = 0;
+            saveLater();
+            maybeQueueOnWalletChanged();
+        } finally {
+            lock.unlock();
+        }
+    }
+
     /**
      * Deletes transactions which appeared above the given block height from the wallet, but does not touch the keys.
      * This is useful if you have some keys and wish to replay the block chain into the wallet in order to pick them up.
@@ -2483,11 +2502,7 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
         lock.lock();
         try {
             if (fromHeight == 0) {
-                unspent.clear();
+                clearTransactions();
-                spent.clear();
-                pending.clear();
-                dead.clear();
-                transactions.clear();
                 saveLater();
             } else {
                 throw new UnsupportedOperationException();
@@ -2497,6 +2512,14 @@ public class Wallet extends BaseTaggableObject implements Serializable, BlockCha
         }
     }
 
+    private void clearTransactions() {
+        unspent.clear();
+        spent.clear();
+        pending.clear();
+        dead.clear();
+        transactions.clear();
+    }
+
     /**
      * Returns all the outputs that match addresses or scripts added via {@link #addWatchedAddress(Address)} or
      * {@link #addWatchedScripts(java.util.List)}.

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

@@ -107,7 +107,8 @@ public interface WalletEventListener extends KeyChainEventListener {
      *     <li>A pending transaction changes confidence due to some non-new-block related event, such as being
      *     announced by more peers or by  a double-spend conflict being observed.</li>
      *     <li>A re-organize occurs. Call occurs only if the re-org modified any of our transactions.</li>
-     *     <li>A new spend is committed to the wallet</li>
+     *     <li>A new spend is committed to the wallet.</li>
+     *     <li>The wallet is reset and all transactions removed.<li>
      * </ol>
      *
      * <p>When this is called you can refresh the UI contents from the wallet contents. It's more efficient to use