Cleanups of probes using new API.

* Cleanup the handler of request mechanisms
* Move common SetupSbtChild code out into shared code between plugins.
* Fix long-standing issue where `uiContext` key was not being filled in with a valid value as we executed tasks.
* Formalize how we hook the build in SetupSbtChild.   Prefer having as many "global" fixes as possible with the least amount of setting rewiring done upon task execution.  Ideally, we'd like to completely avoid rewiring settings during requests.


Review by @havocp cc @harrah @skyluc