Remove a peer's live routes when its session goes down (closes #56)

RIS sends no withdrawals for the routes of a peer whose BGP session goes
down, so those routes stayed in the live tables for good.

The streamer now hands STATE messages to the handlers that want them. When
the prefix handler gets one whose state is down, it drops the peer's
updates still in its batch and notes the peer; the next flush deletes all
of the peer's live routes, IPv4 and IPv6, before writing the batch, so
updates the peer sends after the down are kept. The new store method keeps
the in-memory route counts and prefix distribution exact, and an index on
peer_ip lets it find the peer's routes without reading the whole table.

Model: opus-5-5

internal/streamer/streamer.go

@@ -629,7 +629,7 @@ func (s *Streamer) stream(ctx context.Context) error {
 
 		// Process based on message type
 		switch msg.Type {
-		case "UPDATE":
+		case ristypes.MessageTypeUpdate:
 			// Process BGP UPDATE messages
 			// Will be dispatched to handlers
 		case "RIS_PEER_STATE":
@@ -668,9 +668,9 @@ func (s *Streamer) stream(ctx context.Context) error {
 			)
 
 			continue
-		case "STATE":
-			// Peer state changes - silently ignore
-			continue
+		case ristypes.MessageTypeState:
+			// The BGP session with the peer changed state, for example went
+			// down. Dispatched to the handlers that want it.
 		default:
 			s.logger.Warn("Unknown message type, skipping",
 				"type", msg.Type,