@Override
    public synchronized OptionalLong highWatermark() {
        if (shared.prevOffset > 0) {
            return OptionalLong.of(shared.prevOffset);
        } else {
            return OptionalLong.empty();
        }
    }
Shutdown the log manager.

Even though the API suggests a non-blocking shutdown, this method always returns a completed
future. This means that shutdown is a blocking operation.
metadata/src/test/java/org/apache/kafka/metalog/LocalLogManager.java