Fix single Redis connection being used across all threads (#18135)

* Fix single Redis connection being used across all Sidekiq threads

* Fix tests

lib/mastodon/rack_middleware.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class Mastodon::RackMiddleware
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    @app.call(env)
+  ensure
+    clean_up_sockets!
+  end
+
+  private
+
+  def clean_up_sockets!
+    clean_up_redis_socket!
+    clean_up_statsd_socket!
+  end
+
+  def clean_up_redis_socket!
+    Thread.current[:redis]&.close
+    Thread.current[:redis] = nil
+  end
+
+  def clean_up_statsd_socket!
+    Thread.current[:statsd_socket]&.close
+    Thread.current[:statsd_socket] = nil
+  end
+end