Analytic Functions: BE support for ROWS windows with arbitrary start bounds

Adds support in the BE AnalyticEvalNode for ROWS windows with arbitrary
start bounds. If there is a start bound specified a sliding window must
be maintained. As input rows are processed they are added to the window.
As they expire from the window, they are 'removed' from the current
intermediate state of the evaluators (stored in curr_tuple_) by calling
AggFnEvaluator::Remove(). This is an initial implementation that keeps
the tuples in the window in memory. We can improve this later by using
the BufferedTupleStream with an Iterator interface supporting multiple
readers.

This also fixes IMPALA-1253: LAST_VALUE returns incorrect results

Change-Id: Id5daf6c060ab4079bb8dacf2db8992985894a820
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4335
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>