Prevent ReplicaFetcherThread from throwing UnknownTopicOrPartitionException upon topic creation and deletion.

When does UnknownTopicOrPartitionException typically occur?

• Upon a topic creation, a follower broker of a new partition starts replica fetcher before the prospective leader broker of the new partition receives the leadership information from the controller. Apache Kafka has a an open issue about this (see KAFKA-6221)
• Upon a topic deletion, a follower broker of a to-be-deleted partition starts replica fetcher after the leader broker of the to-be-deleted partition processes the deletion information from the controller.
• As expected, clusters with frequent topic creation and deletion report UnknownTopicOrPartitionException with relatively higher frequency.

What is the impact?

• Exception tracking systems identify the error logs with UnknownTopicOrPartitionException as an exception. This results in a lot of noise for a transient issue that is expected to recover by itself and a natural process in Kafka due to its asynchronous state propagation.

Why not move it to a lower than warn-level log?

• Despite typically being a transient issue, UnknownTopicOrPartitionException may also indicate real issues if it doesn't fix itself after a short period of time. To ensure detection of such scenarios, we set the log level to warn.