PERF: instead of N redis operations perform in bulk

app/services/random_topic_selector.rb

@@ -40,16 +40,18 @@ class RandomTopicSelector
 
     results = []
 
-    left = count
+    return results if count < 1
 
-    while left > 0
+    results = $redis.multi do
-      id = $redis.lpop key
+      $redis.lrange(key, 0, count-1)
-      break unless id
+      $redis.ltrim(key, count, -1)
-
-      results << id.to_i
-      left -= 1
     end
 
+    results = results[0]
+    results.map!(&:to_i)
+
+    left = count - results.length
+
     backfilled = false
     if left > 0
       ids = backfill(category)