Introduce a new AJ scheduler that carries only event_id

Pro:
- refactoring-friendly (i.e. migrating event_type) because event data is
  not stored temporarily in redis, one moving part to care about less
- constant size in redis as opposed to size dependent on data and
  metadata
- simpler configuration (no need for marching scheduler and async
  handler serializers)

Con:
- handlers require database access (they usually do have it)
- additional SQL query to load an event

Serialization cost remains unchanged. Whether passing an id or
full-serialized payload, the data and metadata need to be deserialized.

Also there's no additional serialization step for scheduler, it reuses
serialization for repository if that is needed.

#755

rails_event_store/lib/rails_event_store/active_job_id_only_scheduler.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require "active_job"
+
+module RailsEventStore
+  class ActiveJobIdOnlyScheduler
+    def call(klass, record)
+      klass.perform_later({ "event_id" => record.event_id })
+    end
+
+    def verify(subscriber)
+      Class === subscriber && !!(subscriber < ActiveJob::Base)
+    end
+  end
+end

rails_event_store/lib/rails_event_store/all.rb

@@ -5,6 +5,7 @@
 require_relative "link_by_metadata"
 require_relative "after_commit_async_dispatcher"
 require_relative "active_job_scheduler"
+require_relative 'active_job_id_only_scheduler'
 require_relative "client"
 require_relative "json_client"
 require_relative "version"

rails_event_store/spec/active_job_id_only_scheduler_spec.rb

@@ -0,0 +1,90 @@
+require "spec_helper"
+require "ruby_event_store/spec/scheduler_lint"
+
+module RailsEventStore
+  RSpec.describe ActiveJobIdOnlyScheduler do
+    around do |example|
+      begin
+        original_logger = ActiveJob::Base.logger
+        ActiveJob::Base.logger = nil
+
+        original_adapter = ActiveJob::Base.queue_adapter
+        ActiveJob::Base.queue_adapter = :test
+
+        example.run
+      ensure
+        ActiveJob::Base.logger = original_logger
+        ActiveJob::Base.queue_adapter = original_adapter
+      end
+    end
+
+    before(:each) do
+      MyAsyncHandler.reset
+    end
+
+    it_behaves_like :scheduler, ActiveJobIdOnlyScheduler.new
+    it_behaves_like :scheduler, ActiveJobIdOnlyScheduler.new
+
+    let(:event)  { TimeEnrichment.with(Event.new(event_id: "83c3187f-84f6-4da7-8206-73af5aca7cc8"), timestamp: Time.utc(2019, 9, 30)) }
+    let(:record) { RubyEventStore::Mappers::Default.new.event_to_record(event) }
+
+    describe "#verify" do
+      specify do
+        scheduler      = ActiveJobIdOnlyScheduler.new
+        proper_handler = Class.new(ActiveJob::Base)
+        expect(scheduler.verify(proper_handler)).to eq(true)
+      end
+
+      specify do
+        scheduler  = ActiveJobIdOnlyScheduler.new
+        some_class = Class.new
+        expect(scheduler.verify(some_class)).to eq(false)
+      end
+
+      specify do
+        scheduler = ActiveJobIdOnlyScheduler.new
+        expect(scheduler.verify(ActiveJob::Base)).to eq(false)
+      end
+
+      specify do
+        scheduler = ActiveJobIdOnlyScheduler.new
+        expect(scheduler.verify(Object.new)).to eq(false)
+      end
+    end
+
+    describe "#call" do
+      specify do
+        scheduler = ActiveJobIdOnlyScheduler.new
+        scheduler.call(MyAsyncHandler, record)
+
+        enqueued_jobs = ActiveJob::Base.queue_adapter.enqueued_jobs
+        expect(enqueued_jobs.size).to eq(1)
+        expect(enqueued_jobs[0]).to include(
+          {
+            job: MyAsyncHandler,
+            args: [
+              {
+                "event_id"        => "83c3187f-84f6-4da7-8206-73af5aca7cc8",
+                "_aj_symbol_keys" => [],
+              },
+            ],
+            queue: "default",
+          },
+        )
+      end
+    end
+
+    class MyAsyncHandler < ActiveJob::Base
+      @@received = nil
+      def self.reset
+        @@received = nil
+      end
+      def self.received
+        @@received
+      end
+      def perform(event)
+        @@received = event
+      end
+    end
+  end
+end