Replace middleware with executor callback.

Per request digest caches were added before we had hooks into
different units of work that Rails performs.

As such the most reliable way to prevent stale cache digests
was with a middleware. The middleware prevented staleness in
Action Controller requests.

However, the executor is superior because it should also prevent
staleness when running just Active Job jobs or broadcasting
through Action Cable's server.

actionview/lib/action_view/digestor.rb

@@ -6,13 +6,6 @@ module ActionView
   class Digestor
     @@digest_mutex   = Mutex.new
 
-    class PerRequestDigestCacheExpiry < Struct.new(:app) # :nodoc:
-      def call(env)
-        ActionView::LookupContext::DetailsKey.clear
-        app.call(env)
-      end
-    end
-
     class << self
       # Supported options:
       #

actionview/lib/action_view/railtie.rb

@@ -40,7 +40,7 @@ module ActionView
     initializer "action_view.per_request_digest_cache" do |app|
       ActiveSupport.on_load(:action_view) do
         if app.config.consider_all_requests_local
-          app.middleware.use ActionView::Digestor::PerRequestDigestCacheExpiry
+          app.executor.to_run { ActionView::LookupContext::DetailsKey.clear }
         end
       end
     end