StoredBlock: use Utils to convert chainWork (Backport)

This is a fix for Issue bitcoinj#3410 that will allow main net blocks above
849,137 to be processed. As of block 849,138 we can no longer fit
total chainwork in a 12-byte *signed* field. This fix "kicks the can
down the road" by making the field 12-bytes *unsigned*.

We should open a new issue to address the long term need for bigger
values. Note that converting the field to 12-byte unsigned precludes
us from using the most-significant bit as a flag for a new format, but
we should be able to pick some arbitrary value, say 0xA0 as a version
flag and declare that values less than 0xA0 are "unversioned".

This is BACKPORT to the 0.16 branch. In the master/0.17 branch
the bigIntegerToBytes method has been moved to base.internal.BytUtils,
so this commit differs in that one respect.

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

@@ -117,8 +117,7 @@ public StoredBlock getPrev(BlockStore store) throws BlockStoreException {
 
     /** Serializes the stored block to a custom packed format. Used by {@link CheckpointManager}. */
     public void serializeCompact(ByteBuffer buffer) {
-        byte[] chainWorkBytes = getChainWork().toByteArray();
-        checkState(chainWorkBytes.length <= CHAIN_WORK_BYTES, "Ran out of space to store chain work!");
+        byte[] chainWorkBytes = Utils.bigIntegerToBytes(getChainWork(), CHAIN_WORK_BYTES);
         if (chainWorkBytes.length < CHAIN_WORK_BYTES) {
             // Pad to the right size.
             buffer.put(EMPTY_BYTES, 0, CHAIN_WORK_BYTES - chainWorkBytes.length);