Only update the partition watermark iff the partition watermark in metadata table is smaller than the given watermark.

Author: jiangzzhu

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/spanner/changestreams/dao/PartitionMetadataDao.java

@@ -32,6 +32,7 @@
 import com.google.cloud.Timestamp;
 import com.google.cloud.spanner.DatabaseClient;
 import com.google.cloud.spanner.Dialect;
+import com.google.cloud.spanner.KeySet;
 import com.google.cloud.spanner.Mutation;
 import com.google.cloud.spanner.Options;
 import com.google.cloud.spanner.ResultSet;