Treat a path with AS 0 anywhere as a withdrawal (closes #141)

An AS path that holds AS 0 anywhere, a member of an AS set included, now
decodes as an empty path. RFC 7607 makes such a path malformed and its
announcements withdrawals. An empty path already gets that handling:
the prefix handler deletes the peer's route for the prefix, and the AS
and peering handlers record nothing from it. Before, a path such as
[64496, 0, 64497] stored the route with origin AS64497, recorded both
ASes, and recorded the peerings between the path's other ASes.

Model: opus-5-5

README.md

@@ -195,11 +195,11 @@ 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 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.
+      deletes on withdrawal or on an announcement whose AS path is empty or
+      holds AS 0 anywhere, 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**