…DataFrameWriter methods

QueryExecutionListener has two methods onSuccess() and onFailure() that takes a QueryExecution object as a parameter that gets called when a query is executed. It gets called for several of the DataSet methods like take, head, first, collect etc. but doesn't get called for any of the DataFrameWriter methods like saveAsTable, save etc. This commit fixes this issue and makes calls to these two methods from DataFrameWriter output methods.
Also, added a new property "spark.sql.queryExecutionListeners" that can be used to specify instances of QueryExecutionListeners that should be attached to the SparkSession when the spark application starts up. Testing was done using unit tests.