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/file.go

@@ -18,8 +18,8 @@ var log = clog.NewWithPlugin("file")
 type (
 	// File is the plugin that reads zone data from disk.
 	File struct {
-		Next  plugin.Handler
-		Zones Zones
+		Next plugin.Handler
+		Zones
 	}
 
 	// Zones maps zone names to a *Zone.