Results 2 issues of Viktor Belényesi

- [`children`](https://github.com/bvic23/VinceRP/blob/master/vincerp/Core/Node.swift#L12-L17) gets the current value atomically - some weak references go away due to operations on another thread - [`children.flatMap`](https://github.com/bvic23/VinceRP/blob/master/vincerp/Core/Node.swift#L22) gets the updated value atomically - result: [`descendants`](https://github.com/bvic23/VinceRP/blob/master/vincerp/Core/Node.swift#L20-L25) is...

enhancement