Make saltnado conditionally wait for new events
Remove unecessary delete, on_finish already times-out the futures
Create/Destroy the event listener

This way we don't have to go through the backlog when we get the next event (uses less memory and CPU)
Typo fixes
Pylint fix
Add some text to these exceptions (you shouldn't get them in regular running, i have them here primarily for people that modify the code ;) )
Merge pull request #15572 from jacksontj/saltnado

Updates to rest_tornado