text: Title: Handlers for resuming only unmodified/idling/sleeping resources
  Body: > <a href="https://github.com/nolar"><img align="left" height="50" src="https://avatars0.githubusercontent.com/u/544296?v=4"></a>        An issue by [nolar](https://github.com/nolar) at _2019-11-18 20:48:46+00:00_
  > Original URL: https://github.com/zalando-incubator/kopf/issues/241
  > &nbsp;
  
  ## Background
  
  As discussed in #223 comments (https://github.com/nolar/kopf/issues/223#issuecomment-554996297 and below), there is one type of handlers is missing: when an operator restarts, it should process all the objects that existed before, but were neither updated, nor deleted.
  
  Originally, this was a technical solution for `@kopf.on.resume()` handlers in #96. But then it was intentionally changed to #105 & [0.16](https://github.com/nolar/kopf/releases/tag/0.16) to make the resuming handlers for task/thread spawning, i.e. executed no matter what was the object's state on the operator startup (created/updated/deleted). And later fixed in #230 & [0.23rc1](https://github.com/nolar/kopf/releases/tag/0.23rc1) to be actually executed as intended.
  
  ## Goal
  
  However, the use-case of "unmodified only" handlers is missing now, and cannot be simulated with any combination of the existing handlers.
  
  Find a good name for it. `@kopf.on.notice()`? `@kopf.on.recall()`? `@kopf.on.existence()`? Anything else?
  
  Add such a handler for "unmodified only" cases on the operator startup. 
  
  It must be triggered **only** if CREATE/UPDATE/DELETE cause reasons are **not** applicable. For all these causes, their relevant handlers will be executed, plus on-resume mixed-in handlers.
  
  ## Related
  
  A use-case described for these handlers (https://github.com/nolar/kopf/issues/223#issuecomment-555124183) is basically a reconciliation, and may be related to #150 and #19. 
  
  In this case, however, the resource should be processed on the operator restarts — but should remain separated from the creation/update/deletion handlers for clarity, unlike `@kopf.on.resume()`, which is mixed in with all of them.
  
  ## Implementation hints
  
  This should be easy.  First, see `kopf.reactor.causation.Reason.RESUME` — probably, must be renamed to something else to not be confused with the resuming handlers. Second, add a decorator in `kopf.on` with `reason=WHATEVERITISNAMED, initial=True`. This should be enough. Try in action. Add few tests (look for `on.resume` and `Reason.RESUME` tests). Add the docs.
  
  
  ---
  
  > <a href="https://github.com/pshchelo"><img align="left" height="30" src="https://avatars2.githubusercontent.com/u/1408702?v=4"></a>            Commented by [pshchelo](https://github.com/pshchelo) at _2019-11-19 15:02:25+00:00_
  > &nbsp;
  
  first, thanks for your help with our use-case :+1: 
  
  as for naming, I'd suggest `on.exists`, but whatever floats your boat :-)