Docs: Specify additional info in post-remove for prerouting.

By specifying the sender when adding post removes, post removes can later
be removed after pre-routing by clearing post removes for the same sender.
Docs: Typo fix and clarification in CONTROL_ADD_POST_REMOVE.
Docs: Use MUST NOT instead of SHOULD NOT to clarify behavior.
Tests: Update test_post_removes for prerouting.
MessageDirector: Implement pre-routing postremoves (upstream only).
Merge branch 'master' into feature/preroute-post-removes
Merge pull request #272 from Astron/feature/preroute-post-removes

Feature: Pre-routed Post Removes (Upstream Only)