Cache the last usage data to avoid unicorn timeouts

Closes #1044

CHANGELOG-EE.md

 Please view this file on the master branch, on stable branches it's out of date.
 
 ## 8.13.0 (2016-10-22)
+v 8.13.0 (unreleased)
+  - Cache the last usage data to avoid unicorn timeouts
 
   - Add user activity table and service to query for active users
   - Fix 500 error updating mirror URLs for projects

app/workers/gitlab_usage_ping_worker.rb

@@ -15,7 +15,7 @@ class GitlabUsagePingWorker
 
     begin
       HTTParty.post(url,
-                    body: Gitlab::UsageData.to_json,
+                    body: Gitlab::UsageData.to_json(true),
                     headers: { 'Content-type' => 'application/json' }
                    )
     rescue HTTParty::Error => e

lib/gitlab/usage_data.rb

 module Gitlab
   class UsageData
     class << self
-      def data
-        Rails.cache.fetch('usage_data', expires_in: 1.hour) { uncached_data }
+      def data(force_refresh = false)
+        Rails.cache.fetch('usage_data', force: force_refresh) { uncached_data }
       end
 
       def uncached_data
         license_usage_data.merge(system_usage_data)
       end
 
-      def to_json
-        data.to_json
+      def to_json(force_refresh = false)
+        data(force_refresh).to_json
       end
 
       def system_usage_data