feature/routecheck,ipn/routecheck: probe reachability in the background

Previously, refreshing the routecheck.Client would probe to generate a
new routecheck.Report, but this method was only wired up to the
LocalAPI and the `tailscale routecheck` command. However, waiting for
a probe to finish before choosing a router would take too long, so we
must keep a regularly updated report to be consulted as necessary.

This patch adds a Start and Close method to the routecheck.Client and
starts it in the background from features/routecheck. To enable this
feature for a given node, set both of the following node attributes:
`client-side-reachability` and `client-side-reachability-routecheck`.

This patch also wires up the RouterTracker.OnRoutersChange hook, which
fires a callback whenever a new network map includes information about
a router node, This signals to the routecheck.Client that it might
need to schedule another probe, if the shape of the routing table has
changed materially.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

net/routecheck/report.go

@@ -5,7 +5,6 @@ package routecheck
 
 import (
 	"cmp"
-	"context"
 	"iter"
 	"maps"
 	"net/netip"
@@ -29,17 +28,7 @@ var (
 // It returns nil if a report isn’t available, which happens during initialization.
 func (c *Client) Report() *Report {
 	metricReport.Add(1)
-	nm := c.nm.NetMapNoPeers()
-	if nm == nil {
-		return nil // The report wasn’t available.
-	}
-
-	// TODO(sfllaw): Return the latest snapshot produced by background probing.
-	r, err := c.Refresh(context.TODO(), DefaultTimeout)
-	if err != nil {
-		c.logf("%v", err)
-	}
-	return r
+	return c.report.Load()
 }
 
 // Report contains the result of a single routecheck.