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

* Fix single Redis connection being used across all Sidekiq threads

* Fix tests

spec/lib/delivery_failure_tracker_spec.rb

@@ -22,7 +22,7 @@ describe DeliveryFailureTracker do
 
   describe '#track_failure!' do
     it 'marks URL as unavailable after 7 days of being called' do
-      6.times { |i| Redis.current.sadd('exhausted_deliveries:example.com', i) }
+      6.times { |i| redis.sadd('exhausted_deliveries:example.com', i) }
       subject.track_failure!
 
       expect(subject.days).to eq 7