[PIP-38] Support batch receive in java client. (#4621)
Motivation
Support messages batch receiving, some application scenarios can be made simpler. Users often increase application throughput through batch operations. For example, batch insert or update database.
At present, we provide the ability to receive a single message. If users want to take advantage of batch operating advantages, need to implement a message collector him self. So this proposal aims to provide a universal interface and mechanism for batch receiving messages.
For example:
```
Messages messages = consumer.batchReceive();
insertToDB(messages);
consumer.acknowledge(messages);
Verifying this change
Added new UT to verify this change.
```
1 file changed
