Analytic functions: Initial BE support for ROWS windows

Adds support in the AnalyticEvalNode for ROWS windows with the start
boundary UNBOUNDED PRECEDING, i.e. the end boundary can specify an
offset or CURRENT ROW.

To reduce complexity where we maintain windows and determine when output
results can be produced (ProcessInputBatch), the logic that depends on
the window is factored into several functions. The core functionality
remains the same: for every input row, produce output results if possible,
update the analytic functions, and add the row to the input_stream_ to be
returned later when enough results are available. The functions
TryFinalizePrevRow, TryFinalizeCurrentRow, and InitializeNewPartition
are now called and handle the various window types appropriately.

Change-Id: I36cf76bf11d9e8b48d2556169683abcb43c1db7a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4073
Reviewed-by: Ippokratis Pandis <ipandis@cloudera.com>
Tested-by: jenkins