add sync cleanup method

lib/extensions.js

@@ -26,6 +26,10 @@ module.exports = function(realmConstructor) {
     if (realmConstructor.Sync) {
         realmConstructor.Sync.User = require('./sync').User;
         realmConstructor.Sync.AuthError = require('./errors').AuthError;
+
+        if (realmConstructor.Sync.cleanup) {
+            process.on('exit', () => realmConstructor.Sync.cleanup());
+        }
     }
 
     // TODO: Remove this now useless object.

src/object-store

@@ -1 +1 @@
-Subproject commit f6d64f03c5fd4a1a78770455d5b3dc0f5d2c900d
+Subproject commit 543f9bc748e14b74ee280c50c7a85b74cabaaea5