Default the last_id to 0.

Set `last_id` to 0 which reads from all transactions, instead of -1
which reads only the new transactions from the time of
subscription.

lib/message_bus/client/message_handler.rb

@@ -1,6 +1,6 @@
 module MessageBus::Client::MessageHandler
   SubscribedChannel = Struct.new(:callbacks, :last_id) do
-    def initialize(last_id = -1)
+    def initialize(last_id = 0)
       self.callbacks = []
       self.last_id = last_id
     end