Ruler group execution can overlap with itself after rules update

[bboreham]

When a rules update arrives, scheduler.addNewConfigs() calls addWorkItem(now), and the ruler scheduler de-dupes items added to the queue on the basis of their userD.

But, the item is not in the queue while it is being executed. Thus we can get something like this:

[leth]

It seems like #716 might accidentally fix this?

[bboreham]

@leth I think you're right, in most cases, since the next execution point will be somewhere in the future and the re-add in workItemDone will then de-dupe against it.

One exception would be when the group takes so long to execute that it goes past the next scheduled point. #719 would help that, but again not a complete fix.

[bboreham]

I'm 99% sure this causes an issue every few weeks for our biggest ruleset.