Ensure Flipper memoizer is used in Sidekiq's context

Also, don't use the provided Middleware, which isn't thread-safe, and
instantiate a new Flipper instance per thread instead.
Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/flipper.rb

-require 'flipper/adapters/active_record'
-require 'flipper/adapters/active_support_cache_store'
-
-Flipper.configure do |config|
-  config.default do
-    adapter = Flipper::Adapters::ActiveRecord.new(
-      feature_class: Feature::FlipperFeature, gate_class: Feature::FlipperGate)
-    cached_adapter = Flipper::Adapters::ActiveSupportCacheStore.new(
-      adapter,
-      Rails.cache,
-      expires_in: 1.hour)
-
-    Flipper.new(cached_adapter)
-  end
-end
-
 Feature.register_feature_groups
-
-unless Rails.env.test?
-  require 'flipper/middleware/memoizer'
-  Rails.application.config.middleware.use Flipper::Middleware::Memoizer
-end

lib/feature.rb

+require 'flipper/adapters/active_record'
+require 'flipper/adapters/active_support_cache_store'
+
 class Feature
   # Classes to override flipper table names
   class FlipperFeature < Flipper::Adapters::ActiveRecord::Feature
@@ -60,7 +63,8 @@ class Feature
     end
 
     def flipper
-      @flipper ||= Flipper.instance
+      Thread.current[:flipper] ||=
+        Flipper.new(flipper_adapter).tap { |flip| flip.memoize = true }
     end
 
     # This method is called from config/initializers/flipper.rb and can be used
@@ -68,5 +72,16 @@ class Feature
     # See https://docs.gitlab.com/ee/development/feature_flags.html#feature-groups
     def register_feature_groups
     end
+
+    def flipper_adapter
+      active_record_adapter = Flipper::Adapters::ActiveRecord.new(
+        feature_class: FlipperFeature,
+        gate_class: FlipperGate)
+
+      Flipper::Adapters::ActiveSupportCacheStore.new(
+        active_record_adapter,
+        Rails.cache,
+        expires_in: 1.hour)
+    end
   end
 end