Adding Flow Subscriber APIs to ContextService

[mswatosh]

I'm looking to get feedback on adding these two new APIs to Context Service. After investigation the Flow classes added in Java 9, what I believe would be the most useful addition for Concurrency is providing context on the Flow.Subscriber methods. The new Flow class consists of:

• Flow.Publisher
• Flow.Subscription
• Flow.Subscriber
• Flow.Processor

From what I can tell, Flow.Publisher's methods would generally be invoked where context is available, and Flow.Subscription doesn't have methods where context would add value. Flow.Subscriber is where context could be useful, because it's methods may or may not be called asynchronously. Flow.Processor extends Flow.Publisher and Flow.Subscriber, which is why I provided an API to contextualize it's Flow.Subscriber methods.

Let me know your thoughts, or if there's something else we should be considering here.

[njr-11]

This looks good. I added a suggestion for a minor correction to working and a pointed out another minor item to think about.