ipn/ipnlocal: do not process old status messages received out of order

When `setWgengineStatus` is invoked concurrently from multiple
goroutines, it is possible that the call invoked with a newer status is
processed before a call with an older status. e.g. a status that has
endpoints might be followed by a status without endpoints. This causes
unnecessary work in the engine and can result in packet loss.

This patch adds an `AsOf time.Time` field to the status to specifiy when the
status was calculated, which later allows `setWgengineStatus` to ignore
any status messages it receives that are older than the one it has
already processed.

Updates tailscale/corp#2579

Signed-off-by: Maisem Ali <maisem@tailscale.com>

wgengine/userspace.go

@@ -1113,6 +1113,7 @@ func (e *userspaceEngine) getStatus() (*Status, error) {
 	}
 
 	return &Status{
+		AsOf:       time.Now(),
 		LocalAddrs: append([]tailcfg.Endpoint(nil), e.endpoints...),
 		Peers:      peers,
 		DERPs:      derpConns,