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 routewatch kept them.

The streamer now hands STATE messages to the prefix handler. They are never
dropped for backpressure: one that finds the queue full is held and queued
first once there is room, and the handler's other messages are dropped
meanwhile, so none overtakes it. Nothing waits. On a down, the prefix
handler drops the peer's pending updates and its next flush deletes all the
peer's live routes before the batch's other writes, keeping the in-memory
counts exact. The live route tables are unique on peer and prefix, which
also finds a peer's routes.

Model: opus-5-5

internal/ristypes/ris.go

@@ -6,6 +6,22 @@ import (
 	"time"
 )
 
+// Message types and the session state the handlers act on, as the feed writes
+// them in a message's type and state fields.
+const (
+	// MessageTypeUpdate is a BGP UPDATE: announcements and withdrawals.
+	MessageTypeUpdate = "UPDATE"
+
+	// MessageTypeState is a change in the state of the BGP session between
+	// the collector and the peer.
+	MessageTypeState = "STATE"
+
+	// StateDown is the state of a STATE message whose session has ended: the
+	// collector no longer has any of the peer's routes, and sends no
+	// withdrawals for them.
+	StateDown = "down"
+)
+
 // ASPath represents a BGP AS path as a slice of AS numbers.
 // It handles JSON unmarshaling of both simple arrays and nested AS sets,
 // flattening any nested structures into a single sequence of AS numbers.
@@ -85,6 +101,8 @@ type RISMessage struct {
 	Announcements []RISAnnouncement `json:"announcements,omitempty"`
 	Withdrawals   []string          `json:"withdrawals,omitempty"`
 	Raw           string            `json:"-"`
+	// State is set on STATE messages: the session's new state, such as StateDown.
+	State string `json:"state,omitempty"`
 }
 
 // RISAnnouncement represents a BGP route announcement within a RIS message.