Change from Flow.Subscriber to Pipeline (which extends from Flow.Subscriber)

[rbair23]

We need a few additions to Flow.Subscriber. As a step along that way, convert from using Flow.Subscriber to using Pipeline. We will continue to enhance Pipeline in subsequent PRs. To be honest, I don't really love the name Pipeline, but it is reasonably accurate and alines with the "Pipelines" class name. So I guess it works.