Proposal: split "Start" into user visible "Populate" and "Invoke" stages

[oakad]

This is related to my previous proposal at #666. Same usage scenario: a top level fx App invoking several nested fx Apps.

It will be handy if it was possible to have an explicit App state, whereupon:

1. Constructors were already invoked and Populate options fulfilled.
2. Invoke options and start/stop hooks not yet run.

This will allow the nested fx App to communicate some requirements to the top level fx App via its own constructor (or, at least, it feels to me that it may work).