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

* Fix single Redis connection being used across all Sidekiq threads

* Fix tests

config/initializers/sidekiq.rb

@@ -1,12 +1,12 @@
 # frozen_string_literal: true
 
-require_relative '../../lib/sidekiq_error_handler'
+require_relative '../../lib/mastodon/sidekiq_middleware'
 
 Sidekiq.configure_server do |config|
   config.redis = REDIS_SIDEKIQ_PARAMS
 
   config.server_middleware do |chain|
-    chain.add SidekiqErrorHandler
+    chain.add Mastodon::SidekiqMiddleware
   end
 
   config.server_middleware do |chain|