Handle an announcement without an origin like a replacement (closes #113)

An announcement whose AS path is empty or ends in AS 0 now deletes the
peer's route for the prefix, as a withdrawal does. Before, it was neither
stored nor applied: the peer's old route stayed live, and within one
batch it cancelled the peer's earlier withdrawal of the prefix.

Model: opus-5-5

README.md

@@ -190,10 +190,11 @@ routewatch/
     - **ASHandler**: Tracks all ASNs seen in AS paths
     - **PeerHandler**: Records BGP peer information
     - **PrefixHandler**: Maintains live routing table (upserts on announcement,
-      deletes on withdrawal, deletes all of a peer's routes when its BGP session
-      goes down). Routes are kept per peer IP, so when one peer IP has sessions
-      on two collectors, a session going down on one also removes the routes the
-      other still carries, until they are announced again.
+      deletes on withdrawal or on an announcement whose AS path is empty or ends
+      in AS 0, deletes all of a peer's routes when its BGP session goes down).
+      Routes are kept per peer IP, so when one peer IP has sessions on two
+      collectors, a session going down on one also removes the routes the other
+      still carries, until they are announced again.
     - **PeeringHandler**: Extracts AS peering relationships from AS paths
 
 5. **HTTP Request Flow**