PHJ: remove maintaining tuple row ptrs.

The level of indirection of TupleRow* (which are just a list of Tuple*) is not
helpful. This memory is a waste and additional logic would have to be added to
ensure that it triggers spilling. Instead, we store an offset into the tuple stream
(which is always 8 bytes and already part of the hash table structure).

Change-Id: I89eb9474746a39ba1b0543aafabdc4a8ddccf88b
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4321
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>