Stop OffsetStore when stopping the connector (#12457)

### Motivation

Source connectors based on KCA (all debezium ones) don't stop properly on error / don't restart.
#12441 fixes one problem, this PR fixes another: ofsetStore is not closed on connector stop() and producer/consumer aren't closed too, preventing the connector from shutting down.

### Modifications

Closing offset store on connector stop.

(cherry picked from commit 63454e9)

pulsar-io/kafka-connect-adaptor/src/main/java/org/apache/pulsar/io/kafka/connect/AbstractKafkaConnectSource.java

@@ -180,6 +180,12 @@ public synchronized Record<T> read() throws Exception {
     public void close() {
         if (sourceTask != null) {
             sourceTask.stop();
+            sourceTask = null;
+        }
+
+        if (offsetStore != null) {
+            offsetStore.stop();
+            offsetStore = null;
         }
     }
 

pulsar-io/kafka-connect-adaptor/src/main/java/org/apache/pulsar/io/kafka/connect/PulsarOffsetBackingStore.java

@@ -158,20 +158,30 @@ public void start() {
 
     @Override
     public void stop() {
+        log.info("Stopping PulsarOffsetBackingStore");
         if (null != producer) {
+            try {
+                producer.flush();
+            } catch (PulsarClientException pce) {
+                log.warn("Failed to flush the producer", pce);
+            }
             try {
                 producer.close();
             } catch (PulsarClientException e) {
                 log.warn("Failed to close producer", e);
             }
+            producer = null;
         }
         if (null != reader) {
             try {
                 reader.close();
             } catch (IOException e) {
                 log.warn("Failed to close reader", e);
             }
+            reader = null;
         }
+
+        // do not close the client, it is provided by the sink context
     }
 
     @Override