Write the routing table every second and read it at startup (part of #122)

The prefix handler applies each update to the routing table in memory as
it handles it, sorted into announcements and withdrawals as for the live
route tables. Each flush writes the table's changes first. A failed write
puts its changes and new peers back in the table, so the next write takes
them with each pair's content then. Stop writes the last changes, and
logs those left when its deadline passes.

Startup reads the table before the streamer connects, and logs its pairs,
peers and time. The per-peer counts are a slice, so the read takes about
a third less time. The health check's start period is 60 seconds, so a
full table's read fits before upaas looks.

Model: opus-5-5

internal/routetable/reads.go

@@ -31,6 +31,11 @@ type OriginatedPrefix struct {
 
 // Counts are the table's figures for the stats.
 type Counts struct {
+	// Pairs counts the prefix and origin AS pairs that some peer carries.
+	Pairs int
+	// Peers counts the peers that have a number: those given in
+	// Params.PeerNumbers and those numbered since.
+	Peers        int
 	IPv4Prefixes int
 	IPv6Prefixes int
 	// IPv4PrefixesByLength[n] and IPv6PrefixesByLength[n] are how many
@@ -160,6 +165,7 @@ func (t *Table) Counts() Counts {
 	defer t.mu.RUnlock()
 
 	counts := Counts{
+		Peers:                   len(t.peerNumbers),
 		IPv4PrefixesByLength:    t.ipv4.prefixesByLength(),
 		IPv6PrefixesByLength:    t.ipv6.prefixesByLength(),
 		IPv4Routes:              t.ipv4.routes,
@@ -175,6 +181,10 @@ func (t *Table) Counts() Counts {
 		counts.IPv6Prefixes += n
 	}
 
+	for _, prefixes := range t.byOrigin {
+		counts.Pairs += len(prefixes)
+	}
+
 	oldest, found := t.keptTimes.Min()
 	if found {
 		counts.OldestKeptRoute = &oldest.time