CheckpointManager, BuildCheckpoints: support checkpoints with 32 byte…

… chain work

* `BuildCheckpoints` now always creates checkpoints with 32 byte chain work
* `CheckpointManager` knows how to read both 12 and 32 byte chain work

Also updates the bundle checkpoints to the new format.

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

@@ -129,15 +129,17 @@ private Sha256Hash readTextual(InputStream inputStream) throws IOException {
             checkState(numCheckpoints > 0);
             // Hash numCheckpoints in a way compatible to the binary format.
             hasher.putBytes(ByteBuffer.allocate(4).order(ByteOrder.BIG_ENDIAN).putInt(numCheckpoints).array());
-            final int size = StoredBlock.COMPACT_SERIALIZED_SIZE;
-            ByteBuffer buffer = ByteBuffer.allocate(size);
             for (int i = 0; i < numCheckpoints; i++) {
                 byte[] bytes = BASE64.decode(reader.readLine());
                 hasher.putBytes(bytes);
-                ((Buffer) buffer).position(0);
-                buffer.put(bytes);
-                ((Buffer) buffer).position(0);
-                StoredBlock block = StoredBlock.deserializeCompact(buffer);
+                ByteBuffer buffer = ByteBuffer.wrap(bytes);
+                StoredBlock block;
+                if (bytes.length == StoredBlock.COMPACT_SERIALIZED_SIZE)
+                    block = StoredBlock.deserializeCompact(buffer);
+                else if (bytes.length == StoredBlock.COMPACT_SERIALIZED_SIZE_V2)
+                    block = StoredBlock.deserializeCompactV2(buffer);
+                else
+                    throw new IllegalStateException("unexpected length of checkpoint: " + bytes.length);
                 checkpoints.put(block.getHeader().time(), block);
             }
             HashCode hash = hasher.hash();