mapjoin should throw an error if the input is too large

If the input to the map join is larger than a specific threshold, it may lead to a very slow execution of the join.
It is better to throw an error, and let the user redo his query as a non map-join query.

However, the current map-reduce framework will retry the mapper 4 times before actually killing the job.
Based on a offline discussion with Dhruba, Ning and myself, we came up with the following algorithm:

Keep a threshold in the mapper for the number of rows to be processed for map-join. If the number of rows
exceeds that threshold, set a counter and kill that mapper.

The client (ExecDriver) monitors that job continuously - if this counter is set, it kills the job and also
shows an appropriate error message to the user, so that he can retry the query without the map join.