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 prefix handler and never drops
one for backpressure: one that finds the queue full is held, and queued in
feed order ahead of anything else for that handler once there is room. On a
down, the prefix handler 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 sent after the down are kept.
The live route tables' unique key now leads with peer_ip, so it 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.