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 and
never drops one for backpressure: it waits for room in a full queue until
the idle timer drops the connection. 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 live route tables' unique key now leads with peer_ip, so it also
finds a peer's routes.

Model: opus-5-5

internal/database/schema.sql

@@ -107,7 +107,10 @@ 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, peer_ip) -- a peer has at most one route for a prefix
+	-- A peer has at most one route for a prefix. peer_ip comes first so that
+	-- this key also finds all of a peer's routes, when its BGP session goes
+	-- down; lookups by prefix use the prefix index below.
+	UNIQUE(peer_ip, prefix)
 );
 
 -- IPv6 routing table maintained by PrefixHandler
@@ -120,7 +123,8 @@ 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, peer_ip) -- a peer has at most one route for a prefix
+	-- As in live_routes_v4: peer_ip first, to find all of a peer's routes.
+	UNIQUE(peer_ip, prefix)
 );
 
 -- Indexes for live_routes_v4 table