Yingdac support for append op on wasb hdfs

For certain HDFS implementation such as Azure blob storage wasb protocol, the append operation is not supported. This will fail our attempt to append entry in evaluator activities log on HDFS. 
In this case, we will "simulate" the append operation by 
1) read the entire existing content, 
2) append entry at the end, 
3) delete-and-recreate log file with appended text.