MapJoinCounterHook doesnot work for tez

In HIVE-1792, MapJoinCounterHook hook was added to track joins that get converted to map join. This hook gets List of Tasks from hookContext and check Tag associated with each task. For mr, we create Conditional tasks in case of joins and add tags for respective join conversions. This does not work in case of tez as we only create TezTask(no Conditional Task is created) which can handle multiple joins in contrast to one Conditional Task per join in mr.
The current approach will fail even if we add tag to TezTask as it can have multiple joins of the same type which will require counter.
One possible solution for tez, is to parse query-plan after query completion which we get from hookContext to get workGraph. Using workGraph, we can walk through Operator Tree to find join conversions.

 If this approach looks good, I can raise Pull Request
cc ashutoshc jcamachorodriguez pxiong