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/file/secondary_test.go

@@ -80,7 +80,7 @@ func TestShouldTransfer(t *testing.T) {
 	}
 	defer s.Shutdown()
 
-	z := new(Zone)
+	z := NewZone("testzone", "test")
 	z.origin = testZone
 	z.TransferFrom = []string{addrstr}