Deprecate OperationContext.Stage.IMMEDIATE

Stage.IMMEDIATE is just a shorthand for the 'stage' and 'addFirst' params exposed by the various OperationContext.addStep methods, equivalent to 'the current stage' and 'true'. It produces more understandable code to use, e.g. Stage.MODEL and 'true' than to use a fictitious Stage like IMMEDIATE, so IMMEDIATE should be deprecated as API bloat.

One problem with IMMEDIATE is its name leads people to think the step is executed immediately, i.e. as part of the addStep call. This is incorrect but is a reasonable mistake to make.