clear historyKeeper cache when we trim a channel

lib/commands/channel.js

@@ -105,9 +105,8 @@ Channel.removeOwnedChannelHistory = function (Env, channelId, unsafeKey, hash, c
     }).nThen(function () {
         Env.msgStore.trimChannel(channelId, hash, function (err) {
             if (err) { return void cb(err); }
-
-
-            // XXX you must also clear the channel's index from historyKeeper cache
+            // clear historyKeeper's cache for this channel
+            Env.historyKeeper.channelClose(channelId);
         });
     });
 };