Feature: Pre-routed Post Removes (Upstream Only)

Implements: https://github.com/Astron/Astron/issues/188

Note that this pull request only pre-routes post removes upstream (all the way up to the root).
Conveniently, this functionality is exactly what the docs describe.

To have a more robust system where post removes are also forwarded downstream, first implement: https://github.com/Astron/Astron/issues/271