FEATURE: add a simple queue Scheduler::Defer.later {}

For quick jobs that do not need to be sent to sidekiq,
runs inline in a single thread but does not block

lib/scheduler/defer.rb

@@ -0,0 +1,48 @@
+module Scheduler
+  module Deferrable
+    def initialize
+      @async = Rails.env != "test"
+      @queue = Queue.new
+      @thread = Thread.new {
+        while true
+          do_work
+        end
+      }
+    end
+
+    # for test
+    def async=(val)
+      @async = val
+    end
+
+    def later(&blk)
+      if @async
+        @queue << [RailsMultisite::ConnectionManagement.current_db, blk]
+      else
+        blk.call
+      end
+    end
+
+    def stop!
+      @thread.kill
+    end
+
+    private
+
+    def do_work
+      db, job = @queue.deq
+      RailsMultisite::ConnectionManagement.establish_connection(db: db)
+      job.call
+    rescue => ex
+      Discourse.handle_exception(ex)
+    ensure
+      ActiveRecord::Base.connection_handler.clear_active_connections!
+    end
+
+  end
+
+  class Defer
+    extend Deferrable
+    initialize
+  end
+end