Add a delay before we start polling the connectivity check endpoint to avoid tightlooping if the conn check succeeds but /sync etc fails.

lib/sync.js

@@ -382,7 +382,7 @@ SyncApi.prototype.stop = function() {
  */
 SyncApi.prototype.retryImmediately = function() {
     if (!this._connectionReturnedDefer) { return false; }
-    this._startKeepAlives();
+    this._startKeepAlives(0);
     return true;
 };
 
@@ -686,13 +686,25 @@ SyncApi.prototype._processSyncResponse = function(syncToken, data) {
 };
 
 /**
+ * Starts polling the connectivity check endpoint
+ * @param {number} delay How long to delay until the first poll.
+ *        defaults to a short, randomised interval (to prevent
+ *        tightlooping if /versions succeeds but /sync etc. fail).
  * @return {promise}
  */
-SyncApi.prototype._startKeepAlives = function() {
+SyncApi.prototype._startKeepAlives = function(delay) {
+    if (delay === undefined) {
+        delay = 5000 + Math.floor(Math.random() * 5000)
+    }
+
     if (this._keepAliveTimer !== null) {
         clearTimeout(this._keepAliveTimer);
     }
-    this._pokeKeepAlive();
+    var self = this;
+    self._keepAliveTimer = setTimeout(
+        self._pokeKeepAlive.bind(self),
+        delay
+    );
     if (!this._connectionReturnedDefer) {
         this._connectionReturnedDefer = q.defer();
     }
@@ -948,7 +960,7 @@ SyncApi.prototype._updateSyncState = function(newState, data) {
  */
 SyncApi.prototype._onOnline = function() {
     debuglog("Browser thinks we are back online");
-    this._startKeepAlives();
+    this._startKeepAlives(0);
 };
 
 function createNewUser(client, userId) {