plugin/file: load secondary zones lazily on startup (#2944)

This fixes a long standing bug:
fixes: #1609

Load secondary zones in a go-routine; this required another mutex to
protect some fields; I think those were needded anyway because a
transfer can also happen when we're running; we just didn't have a test
for that situation.

The test had to be changed to wait for the transfer to happen at this is
async now.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/secondary/setup.go

@@ -29,8 +29,8 @@ func setup(c *caddy.Controller) error {
 		if len(z.TransferFrom) > 0 {
 			c.OnStartup(func() error {
 				z.StartupOnce.Do(func() {
-					z.TransferIn()
 					go func() {
+						z.TransferIn()
 						z.Update()
 					}()
 				})