FIX: redis leak when visiting large amounts of topics

Message bus uses a key to keep track of the last id for each channel
this key was never expired even if channel data expired

This change ensures we also expire the tracking key, it means a lot to us
cause each topic has a channel, so if you have 1 million topics that is
1 million keys that may persist forever

Gemfile

@@ -43,7 +43,12 @@ gem 'ember-source', '2.13.3'
 gem 'ember-handlebars-template', '0.7.5'
 gem 'barber'
 
-gem 'message_bus'
+# message bus 2.2.0 should be very stable
+# we trimmed some of the internal API surface down so we went with
+# a pre release here to make we don't do a full release prior to
+# baking here. Remove 2.2.0.pre no later than Jan 2019 and move back
+# to the standard releases
+gem 'message_bus', '2.2.0.pre.1'
 
 gem 'rails_multisite'
 

Gemfile.lock

@@ -193,7 +193,7 @@ GEM
       mini_mime (>= 0.1.1)
     maxminddb (0.1.21)
     memory_profiler (0.9.12)
-    message_bus (2.1.6)
+    message_bus (2.2.0.pre.1)
       rack (>= 1.1.3)
     metaclass (0.0.4)
     method_source (0.8.2)
@@ -491,7 +491,7 @@ DEPENDENCIES
   mail (= 2.7.1.rc1)
   maxminddb
   memory_profiler
-  message_bus
+  message_bus (= 2.2.0.pre.1)
   mini_mime
   mini_racer
   mini_scheduler