Make runnable group's Start method configurable to block until runnables Start method exit.

[godwinpang]

runnablegroup.Start currently returns once the runnables in the group have been started. There is a new use case in #3192 where we want to block runnablegroup.Start until the runnablegroup's runnables Start method exit. The only way of doing so today is to add a separate method to track each runnable's completion and hook it as a check into the runnablegroup.

Handwavey proposal is either to

• Introduce a new BlockingRunnableGroup.
• Make the Start blocking behavior configurable.

Context: #3192 (comment)_