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/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