prevent server overload due to a tsunami of AO cache preloading (prob…

…ably) due to some other plugin calling autoptimizeCache::clearall, cfr. https://wordpress.org/support/topic/many-cachebuster-requests/ and https://wordpress.org/support/topic/autooptimize-slows-down-site-after-upgrade/

classes/autoptimizeCache.php

@@ -105,7 +105,8 @@ function autoptimize_do_cachepurged_action() {
         add_action("autoptimize_action_cachepurged","autoptimize_flush_pagecache",10,0);
 
         // warm cache (part of speedupper)?
-        if ( apply_filters('autoptimize_filter_speedupper', true) ) {
+        if ( apply_filters('autoptimize_filter_speedupper', true) && get_transient('autoptimize_warmcache_buster') == false ) {
+            set_transient('autoptimize_warmcache_buster', 'please bust me next time', HOUR_IN_SECONDS);
             $warmCacheUrl = site_url()."/?ao_speedup_cachebuster=".rand(1,100000);
             $warmCache = @wp_remote_get($warmCacheUrl);
             unset($warmCache);