Pipeline Expressions Refactor

[tom-andersen]

• Make NULL_VALUE constant as an optimization against create the same object over and over again.
• Make Expr polymorphic with toProto() method. This eliminates large condition structure. To toProto() is package-private for internal use. Package-private is only possible on classes (not interfaces), so I converted the Expr to an abstract class with package-private constructor to avoid customer extending it.
• Remove type parameters and use ? instead.
• Fix operators that take Object as parameter. Object might be of type Expr which cannot be passed through Constant.of(..) method. Instead we need to short circuit conversion, and simply return Object since it is already an Expr.