Log DataNodes in the write pipeline

Trying to diagnose a slow HDFS flush, taking longer than 10 seconds, but did not know which DNs were involved in the write pipeline. Of course, I could search NN log for the list of DNs, but it might not be possible sometimes or convenient.

Propose to add a DEBUG trace to DataStreamer#setPipeline to print the list of DNs in the pipeline.

BTW, we do print the list of DNs during pipeline recovery.