schedule fate threads after fate construction

**Is your feature request related to a problem? Please describe.**
Would be safer to construct the Fate objects then call something like `fate.start()`. Right now, some of the fate threads will be started after a delay, but are scheduled in the constructor. Should be scheduled after construction to avoid any potential problems like leaking `this` before construction if the threads somehow run before construction completes.

**Describe the solution you'd like**
fate threads are scheduled in a new method like `start()` instead of in the constructor. Will need to change all uses of Fate to also call start.

**Additional context**
Fate constructor is currently leaking `this` with `startFateExecutors()`, but this is being removed in #5817.
This ticket should wait until that PR is merged.