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

A peer has at most one route per prefix, but both live route tables were
unique on prefix, origin AS and peer. A peer that changed a prefix's origin
AS left its old route behind, and a withdrawal sent in the same message as
announcements was given their origin AS and missed the withdrawn route.

Both tables are now unique on prefix and peer. An announcement replaces the
peer's row for the prefix, origin AS included; a withdrawal deletes it
without looking at an origin AS. The route counts and prefix distribution
kept in memory stay exact, for IPv4 and IPv6.

Model: opus-5-5

internal/database/models.go

@@ -114,7 +114,6 @@ type IPInfo struct {
 // LiveRouteDeletion represents parameters for deleting a live route
 type LiveRouteDeletion struct {
 	Prefix    string
-	OriginASN int
 	PeerIP    string
 	IPVersion int
 }