KAFKA-7873; Always seek to beginning in KafkaBasedLog (#6203)

Explicitly seek KafkaBasedLog’s consumer to the beginning of the topic partitions, rather than potentially use committed offsets (which would be unexpected) if group.id is set or rely upon `auto.offset.reset=earliest` if the group.id is null.

This should not change existing behavior but should remove some potential issues introduced with KIP-287 if `group.id` is not set in the consumer configurations. Note that even if `group.id` is set, we still always want to consume from the beginning.

Reviewers: Jason Gustafson <jason@confluent.io>

connect/runtime/src/main/java/org/apache/kafka/connect/util/KafkaBasedLog.java

@@ -146,6 +146,10 @@ public class KafkaBasedLog<K, V> {
             partitions.add(new TopicPartition(partition.topic(), partition.partition()));
         consumer.assign(partitions);
 
+        // Always consume from the beginning of all partitions. Necessary to ensure that we don't use committed offsets
+        // when a 'group.id' is specified (if offsets happen to have been committed unexpectedly).
+        consumer.seekToBeginning(partitions);
+
         readToLogEnd();
 
         thread = new WorkThread();