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.go

@@ -51,9 +51,11 @@ Transfer:
 		return Err
 	}
 
+	z.apexMu.Lock()
 	z.Tree = z1.Tree
 	z.Apex = z1.Apex
 	*z.Expired = false
+	z.apexMu.Unlock()
 	log.Infof("Transferred: %s from %s", z.origin, tr)
 	return nil
 }