While thinking through issues that might cause #25718 I ran into this case-- where you can create 2 sync wrappers with the same parent IOLoop and run into problems because you can't start/stop the loop multiple times like that and have the call tree work correctly. To solve this (since it *should* be the exceptional case) I'm making SyncWrapper keep track of which loops are in use-- then if someone attempts this situation it will simply make another one.
