Analytic Functions: Add rank() and dense_rank()

Adds the rank() and dense_rank() analytic functions and makes internal
changes to the AggFnEvaluator that are necessary to support calling
Finalize() repeatedly (as the AnalyticEvalNode does) on UDAs that destroy
state in Finalize().

Rank requires both the current rank and the count at that rank in order to
determine the next rank, so the intermediate state is a StringVal containing
a struct with these two fields.

Aggregate functions (internally only, for now) can expose a GetValue() method
which takes an intermediate value and returns a final value without destroying
the intermediate state. Finalize() is then used to clean up intermediate
state, if necessary.

This also adds a second optional, internal-only function for UDAs to allow
removing values from intermediate state: Remove(). This will be required for
implementing sliding windows later but is added here because the change is
nearly identical to that for adding GetValue().

Some cleanup in the AnalyticEvalNode, most notably we avoid allocating tuples
to DeepCopy prev_input_row_ between input batches. Instead, we keep the last
two child row batches because the prev child row batch owns the resources for
prev_input_row_.

Change-Id: I5a30eb517a38d369fe63f7af91904a4b9786fadc
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3962
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins