Fix some performance issues with /admin/instances (#21907)

/admin/instances?availability=failing remains wholly unefficient

app/controllers/admin/instances_controller.rb

@@ -57,7 +57,7 @@ module Admin
     end
 
     def preload_delivery_failures!
-      warning_domains_map = DeliveryFailureTracker.warning_domains_map
+      warning_domains_map = DeliveryFailureTracker.warning_domains_map(@instances.map(&:domain))
 
       @instances.each do |instance|
         instance.failure_days = warning_domains_map[instance.domain]

app/lib/delivery_failure_tracker.rb

@@ -65,8 +65,13 @@ class DeliveryFailureTracker
       domains - UnavailableDomain.all.pluck(:domain)
     end
 
-    def warning_domains_map
-      warning_domains.index_with { |domain| redis.scard(exhausted_deliveries_key_by(domain)) }
+    def warning_domains_map(domains = nil)
+      if domains.nil?
+        warning_domains.index_with { |domain| redis.scard(exhausted_deliveries_key_by(domain)) }
+      else
+        domains -= UnavailableDomain.where(domain: domains).pluck(:domain)
+        domains.index_with { |domain| redis.scard(exhausted_deliveries_key_by(domain)) }.filter { |_, days| days.positive? }
+      end
     end
 
     private