Feature/restart finished demultiplexing

- Implemented functionality for removing the "found" file for processing units (a.k.a. "forgetting" the demultiplexing of that unit)
- Went over the state change events in SisyphusMaster and made them consistent
- Wrote tests for the IlluminaProcessingUnit[Executor]Actor to complement the forget functionality 
- Added a "ProcessingUnitPlaceholder" class, which can be passed to a ProcessingUnitWatcher and eventually exchanged for a real ProcessingUnit if it is available to the watcher.

The comment is a bit misleading. The request to forget the unit is acknowledged and that is what is seen by the api. This means that this request is "queued" by the master in the messagesNotYetProcessed state set and will be processed once a ProcessingUnitWatcher requests some work. If this fails, the master will keep it in the failed messages set but the api/user won't see that until the user sends an api call asking for the status.