wgengine, magicsock, tstun: don't regularly STUN when idle (mobile only for now)

If there's been 5 minutes of inactivity, stop doing STUN lookups. That
means NAT mappings will expire, but they can resume later when there's
activity again.

We'll do this for all platforms later.

Updates tailscale/corp#320

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg.go

@@ -525,6 +525,11 @@ type Debug struct {
 	// LogHeapURL is the URL to POST its heap pprof to.
 	// Empty means to not log.
 	LogHeapURL string `json:",omitempty"`
+
+	// ForceBackgroundSTUN controls whether magicsock should
+	// always do its background STUN queries (see magicsock's
+	// periodicReSTUN), regardless of inactivity.
+	ForceBackgroundSTUN bool `json:",omitempty"`
 }
 
 func (k MachineKey) String() string                   { return fmt.Sprintf("mkey:%x", k[:]) }