Give api server operation tracking ability

Pending and recently completed operations now served at: /api/v1beta1/operations/id#

Handling of async operations is now consistent and sensible (ish). We check for admissibility before starting a long wait. I added some ID != "" checks here, but probably more things should be checked.
It's only written to here in this function, and it's as the last thing the goroutine does, and there is now always a reader. I'd rather leave as-is to explicitly show that it has these properties.