Spread channel saves across the save interval

Since all channels were saved sequentially, this would cause huge lag
spikes every time the channel save interval fired.  This change adds a
delay between each channel so that the additional load is spread evenly
across the save interval.

package.json

@@ -2,7 +2,7 @@
   "author": "Calvin Montgomery",
   "name": "CyTube",
   "description": "Online media synchronizer and chat",
-  "version": "3.16.0",
+  "version": "3.16.1",
   "repository": {
     "url": "http://github.com/calzoneman/sync"
   },

src/bgtask.js

@@ -62,12 +62,15 @@ function initChannelDumper(Server) {
     var CHANNEL_SAVE_INTERVAL = parseInt(Config.get("channel-save-interval"))
                                 * 60000;
     setInterval(function () {
+        var wait = CHANNEL_SAVE_INTERVAL / Server.channels.length;
         Promise.reduce(Server.channels, (_, chan) => {
+            return Promise.delay(wait).then(() => {
                 if (!chan.dead && chan.users && chan.users.length > 0) {
                     return chan.saveState().catch(err => {
                         Logger.errlog.log(`Failed to save /r/${chan.name}: ${err.stack}`);
                     });
                 }
+            });
         }, 0);
     }, CHANNEL_SAVE_INTERVAL);
 }