AbstractPipeline invokes overridden method in constructor

Currently java.util.stream.AbstractPipeline's constructor calls the designed-to-be-overridden `opIsStateful()` which means that since the AbstractPipeline constructor runs before the subclass' initialization logic, that means that any implementations of `opIsStateful()` must only rely on global state and/or constants.

Furthermore, `opIsStateful()` is only used for parallel evaluation, and currently requires a boolean field per AbstractPipeline (`hasAnyStateful`) regardless of whether they ever need to evaluate in parallel or not.

It should be possible to avoid needing the `hasAnyStateful` field, as well as avoiding to call `opIsStateful()` in the AbstractPipeline constructor, so that implementations thereof can use instance state to produce their values, and only invoke it for parallel evaluations.