always delete the throttled cleanup functions once they have run

lib/env.js

@@ -121,9 +121,9 @@ module.exports.create = function (config) {
 
     Env.checkCache = function (channel) {
         var f = Env.cache_checks[channel] || Util.throttle(function () {
+            delete Env.cache_checks[channel];
             if (Env.channel_cache[channel]) { return; }
             delete Env.metadata_cache[channel];
-            delete Env.cache_checks[channel];
         }, 30000);
         f();
     };