Parse NULLIF as a plain routine invocation instead of transforming to IF.

Minor refactoring to reduce duplication of creating routine call nodes
and looking forward to removing JavaToSQL/SQLToJava nodes.
merge nathanlws: Parse NULLIF as a plain routine invocation

Merge pull request #159 from nathanlws/sql-parser/nullif-as-routine
https://github.com/FoundationDB/sql-parser/pull/159
Add a couple new isolation levels for what FDB can practically do.