In cmd broadcast mode blocking command submission (activated by setting the "secondsToWaitForCompletion" query param) has a race condition. The first submission which is completed to a PG backend will call the callback that the client is waiting on and return a result based that submission alone. If one of the submissions happens to fail before any can succeed we will return a 503 to the client even though the command may eventually be stored successfully in one of the PG instances.
In cmd broadcast mode we treat a cmd as successfully submitted if it makes it to at least one PG backend. We should move the callback handling a level up from command.clj/attempt-exec-command so that the callback is called when the first PG submission is successful or if all attempted submissions have failed.
