[ISSUES 55] MessageReceipt should have getMessageId and getOffset API

(openmessaging#55)

Author: odbozhou

openmessaging-api/src/main/java/io/openmessaging/consumer/MessageReceipt.java

@@ -17,11 +17,41 @@
 
 package io.openmessaging.consumer;
 
+import io.openmessaging.message.Header;
+
 /**
  * A {@code MessageReceipt} is a {@code Message} with a {@code Receipt}.
  *
  * @version OMS 1.0.0
  * @since OMS 1.0.0
  */
 public interface MessageReceipt {
+    /**
+     * {@code OFFSET} represents this message offset in partition.
+     * <p>
+     *
+     * @param offset The offset in the current partition, used to quickly get this message in the queue
+     */
+    void setOffset(long offset);
+
+    /**
+     * This method will return the {@code OFFSET}  in the partition to which the message belongs to, but the premise is
+     * that the implementation of the server side is dependent on the partition or a queue-like storage mechanism.
+     *
+     * @return The offset of the partition to which the message belongs.
+     */
+    long getOffset();
+
+    /**
+     * The {@code MESSAGE_ID} header field contains a value that uniquely identify each message sent by a {@code
+     * Producer}. this identifier is generated by producer.
+     */
+    void setMessageId(String messageId);
+
+    /**
+     * See {@link Header#setMessageId(String)}
+     *
+     * @return messageId
+     */
+    String getMessageId();
 }