Currently we support Lead/Lag as navigation UDFs usable with Windowing.
To be standard compliant we need to support Lead & Lag UDAFs.
Will continue to support Lead/Lag UDFs as arguments to UDAFs when Windowing is in play.
Currently allow Lead/Lag expressions to appear in SelectLists even when they are not arguments to UDAFs. Support for this feature will probably be removed. Causes ambiguities when Query contains different partition clauses. Will provide more details with associated Jira to remove this feature.