Unify NULLIF scalar, IFNULL and IF argument type resolution to picking.

That is, like COALESCE.

Adds NullIf, moves MIfNull to IfNull and removes MIfElse.
merge nathanlws: Unify NULLIF scalar, IFNULL and IF argument type resolution to picking.

Merge pull request #1571 from nathanlws/sql-layer/unify-if-ifnull-nullif-arguments
https://github.com/FoundationDB/sql-layer/pull/1571