Inverse children stopping order.

[nightwolfz]

Stopping an actors children should happen in reverse of spawning order.

Example

Let's say actor root spawns the following actors in this order:

1. dbActor
2. reader
3. writer

Both reader and writer use the dbActor in order to save their state. If we poison root, reader and writer won't be able to save their state. I suggest we inverse the order of stopping to make this common use-case feasible.

3. writer
4. reader
5. dbActor