added sync mutex for consistency checks and backup

services/sync_mutex.js

@@ -0,0 +1,8 @@
+/**
+ * Sync makes process can make data intermittently inconsistent. Processes which require strong data consistency
+ * (like consistency checks) can use this mutex to make sure sync isn't currently running.
+ */
+
+const Mutex = require('async-mutex').Mutex;
+
+module.exports = new Mutex();