Analysis error for min()/max() w/ analytic windows without UNBOUNDED PRECEDING

min()/max() do not currently support windows without UNBOUNDED PRECEDING,
so this changes AnalyticExpr to detect this during analysis and throw an
AnalysisException.

Also removed some stale TODOs in the BE

Change-Id: I734b0a5d5399f9bb9d4db6ab1ddc079237b0ac03
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4431
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins