java_gen: renamed OFChecksum128 to U128 + implement HashValue

This has the same semantics as our other Uxx types.
diff --git a/test_data/of13/bsn_gentable_bucket_stats_reply.data b/test_data/of13/bsn_gentable_bucket_stats_reply.data
index 8e0772f..84a36a6 100644
--- a/test_data/of13/bsn_gentable_bucket_stats_reply.data
+++ b/test_data/of13/bsn_gentable_bucket_stats_reply.data
@@ -22,8 +22,8 @@
 builder.setXid(0x12345678)
     .setEntries(
         ImmutableList.<OFBsnGentableBucketStatsEntry>of(
-            factory.bsnGentableBucketStatsEntry(OFChecksum128.of(0x8877665544332211L, 0xFFEEDDCCBBAA9988L)),
-            factory.bsnGentableBucketStatsEntry(OFChecksum128.of(0x1234234534564567L, 0x56786789789A89ABL))
+            factory.bsnGentableBucketStatsEntry(U128.of(0x8877665544332211L, 0xFFEEDDCCBBAA9988L)),
+            factory.bsnGentableBucketStatsEntry(U128.of(0x1234234534564567L, 0x56786789789A89ABL))
         )
     )
 -- c