Identify a live route by prefix and peer (closes #55)

A peer has at most one route for a prefix, but the live route tables were
keyed by prefix, origin AS and peer. An announcement with a new origin AS
added a second row instead of replacing the first, and a withdrawal sent in
the same message as announcements was given their origin AS and so missed
the withdrawn route.

The tables are now unique on prefix and peer. An announcement updates the
peer's row for the prefix, origin AS included, and a withdrawal deletes it
without looking at an origin AS. The origin-only delete statements, the
deletion's origin field and DeleteLiveRoute's origin parameter are removed.

Model: opus-5-5

internal/database/schema.sql

@@ -107,7 +107,7 @@ CREATE TABLE IF NOT EXISTS live_routes_v4 (
 	as_path TEXT NOT NULL, -- JSON array
 	next_hop TEXT NOT NULL,
 	last_updated DATETIME NOT NULL,
-	UNIQUE(prefix, origin_asn, peer_ip)
+	UNIQUE(prefix, peer_ip) -- a peer has at most one route for a prefix
 );
 
 -- IPv6 routing table maintained by PrefixHandler
@@ -120,7 +120,7 @@ CREATE TABLE IF NOT EXISTS live_routes_v6 (
 	as_path TEXT NOT NULL, -- JSON array
 	next_hop TEXT NOT NULL,
 	last_updated DATETIME NOT NULL,
-	UNIQUE(prefix, origin_asn, peer_ip)
+	UNIQUE(prefix, peer_ip) -- a peer has at most one route for a prefix
 );
 
 -- Indexes for live_routes_v4 table