## What changes were proposed in this pull request?
KafkaContinuousSinkSuite leaks a Kafka cluster because both KafkaSourceTest and KafkaContinuousSinkSuite create a Kafka cluster but `afterAll` only shuts down one cluster. This leaks a Kafka cluster and causes that some Kafka thread crash and kill JVM when SBT is trying to clean up tests.
This PR fixes the leak and also adds a shut down hook to detect Kafka cluster leak.
In additions, it also fixes `AdminClient` leak and cleans up cached producers (When a record is writtn using a producer, the producer will keep refreshing the topic and I don't find an API to clear it except closing the producer) to eliminate the following annoying logs:
```
8/13 15:34:42.568 kafka-admin-client-thread | adminclient-4 WARN NetworkClient: [AdminClient clientId=adminclient-4] Connection to node 0 could not be established. Broker may not be available.
18/08/13 15:34:42.570 kafka-admin-client-thread | adminclient-6 WARN NetworkClient: [AdminClient clientId=adminclient-6] Connection to node 0 could not be established. Broker may not be available.
18/08/13 15:34:42.606 kafka-admin-client-thread | adminclient-8 WARN NetworkClient: [AdminClient clientId=adminclient-8] Connection to node -1 could not be established. Broker may not be available.
18/08/13 15:34:42.729 kafka-producer-network-thread | producer-797 WARN NetworkClient: [Producer clientId=producer-797] Connection to node -1 could not be established. Broker may not be available.
18/08/13 15:34:42.906 kafka-producer-network-thread | producer-1598 WARN NetworkClient: [Producer clientId=producer-1598] Connection to node 0 could not be established. Broker may not be available.
```
I also reverted b5eb54244e introduced by #22097 since it doesn't help.
## How was this patch tested?
Jenkins
Closes#22106 from zsxwing/SPARK-25116.
Authored-by: Shixiong Zhu <zsxwing@gmail.com>
Signed-off-by: Shixiong Zhu <zsxwing@gmail.com>
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