fix(subs): key collision on redis

[michael-0acf4]

Follow up of #863
When multiple start occurs for redis, some schedules can happen exactly at the same time resulting into the same identifier (and leading to an inconsistent state).

This solution simply combines the schedule with the run_id making it unique instead of using it as is.

mutation AllAtOnce {
  a: start_retry(kwargs: { .. }) # => calls add_schedule( ... date ...)
  b: start_retry(kwargs: { .. })
  c: start_retry(kwargs: { .. })
  d: start_retry(kwargs: { .. }) 
  e: start_retry(kwargs: { .. })
  f: start_retry(kwargs: { .. })
 # ..
}

Migration notes

None

• The change comes with new or modified tests
• Hard-to-understand functions have explanatory comments
• End-user documentation is updated to reflect the change