Add event_store_locator option to AsyncHandler

Because eager loading of the event store is not suitable to
legacy applications. Howevery current event_store: option will be
removed only at RES 3.0.

rails_event_store/lib/rails_event_store/async_handler_helpers.rb

@@ -6,9 +6,10 @@ def self.with_defaults
       with
     end
 
-    def self.with(event_store: Rails.configuration.event_store, serializer: YAML)
+    def self.with(event_store: Rails.configuration.event_store, event_store_locator: nil, serializer: YAML)
       Module.new do
         define_method :perform do |payload|
+          event_store = event_store_locator.call if event_store_locator
           super(event_store.deserialize(serializer: serializer, **payload.symbolize_keys))
         end
       end

rails_event_store/spec/async_handler_helpers_spec.rb

@@ -18,6 +18,12 @@ def perform(event)
     end
   end
 
+  class HandlerWithEventStoreLocator < ActiveJob::Base
+    def perform(event)
+      $queue.push(event)
+    end
+  end
+
   class HandlerWithSpecifiedSerializer < ActiveJob::Base
     def perform(event)
       $queue.push(event)
@@ -101,6 +107,13 @@ def verify(subscriber)
       expect($queue.pop).to eq(ev)
     end
 
+    specify "with specified event store locator" do
+      HandlerWithEventStoreLocator.prepend RailsEventStore::AsyncHandler.with(event_store: nil, event_store_locator: -> { another_event_store })
+      another_event_store.subscribe_to_all_events(HandlerWithEventStoreLocator)
+      another_event_store.publish(ev = RailsEventStore::Event.new)
+      expect($queue.pop).to eq(ev)
+    end
+
     specify "with specified serializer" do
       HandlerWithSpecifiedSerializer.prepend RailsEventStore::AsyncHandler.with(event_store: json_event_store, serializer: JSON)
       json_event_store.subscribe_to_all_events(HandlerWithSpecifiedSerializer)