[HUDI-328] Adding delete api to HoodieWriteClient (#1004)

[HUDI-328]  Adding delete api to HoodieWriteClient and Spark DataSource

hudi-common/src/main/java/org/apache/hudi/common/model/EmptyHoodieRecordPayload.java

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hudi.common.model;
+
+import org.apache.avro.Schema;
+import org.apache.avro.generic.GenericRecord;
+import org.apache.avro.generic.IndexedRecord;
+import org.apache.hudi.common.util.Option;
+
+/**
+ * Empty payload used for deletions
+ */
+public class EmptyHoodieRecordPayload implements HoodieRecordPayload<EmptyHoodieRecordPayload> {
+
+  public EmptyHoodieRecordPayload() {
+  }
+
+  public EmptyHoodieRecordPayload(GenericRecord record, Comparable orderingVal) {
+  }
+
+  @Override
+  public EmptyHoodieRecordPayload preCombine(EmptyHoodieRecordPayload another) {
+    return another;
+  }
+
+  @Override
+  public Option<IndexedRecord> combineAndGetUpdateValue(IndexedRecord currentValue, Schema schema) {
+    return Option.empty();
+  }
+
+  @Override
+  public Option<IndexedRecord> getInsertValue(Schema schema) {
+    return Option.empty();
+  }
+}

hudi-common/src/main/java/org/apache/hudi/common/model/HoodieCommitMetadata.java

@@ -40,6 +40,7 @@ import org.apache.log4j.Logger;
 @JsonIgnoreProperties(ignoreUnknown = true)
 public class HoodieCommitMetadata implements Serializable {
 
+  public static final String SCHEMA_KEY = "schema";
   private static volatile Logger log = LogManager.getLogger(HoodieCommitMetadata.class);
   protected Map<String, List<HoodieWriteStat>> partitionToWriteStats;
   protected Boolean compacted;