tickets/DM-199: Develop new master-worker result system

Also, in `markCompleted()` you have code like this (executed while holding a lock on `_receiversMutex`):
```c++
if(!success) {
    if(i != _receivers.end()) {
    } else {
        assert(i != _receivers.end());
    }
}
```
Won't the assert always crash the program?