plugin/file: shutdown reload goroutine (#1571)

* plugin/file: shutdown reload goroutine

Shutdown the z.Reload() routine (if started in the first place) on
shutdow and reload.

Fixes #1508

* Must be put in c.OnShutdown()

* up test coverage

plugin/file/setup_test.go

@@ -53,6 +53,20 @@ func TestFileParse(t *testing.T) {
 			false,
 			Zones{Names: []string{"10.in-addr.arpa."}},
 		},
+		{
+			`file ` + zoneFileName1 + ` example.net. {
+				upstream a
+			}`,
+			true,
+			Zones{Names: []string{}},
+		},
+		{
+			`file ` + zoneFileName1 + ` example.net. {
+				no_rebloat
+			}`,
+			true,
+			Zones{Names: []string{}},
+		},
 	}
 
 	for i, test := range tests {