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
Extend tracking of isolation levels through to the transaction service, where something interesting might happen.
Centralize selection of transaction scan options for operator.
Hook those together so that SET TRANSACTION ISOLATION LEVEL selects appropriate scan options for operators.
Allow getting scan options without a transaction for tests that create cursors and only then begin before opening them.