Use ruby cache for stats helper methods

These methods are only used while stats are being generated; once stats
are generated, they aren't used anymore. So there's no need to store
them using the Dalli cache.

Furthermore, there are polls (and even budgets) with hundreds of
thousands of participants. Calculating stats for them takes a very long
time because we can't store all those records in the Dalli cache.

However, since these records aren't used once the stats are generated,
we can store them in an instance variable while we generate the stats,
speeding up the process.

app/models/poll/stats.rb

@@ -96,15 +96,14 @@ class Poll::Stats
     end
 
     def voters
-      poll.voters
+      @voters ||= poll.voters
     end
 
     def recounts
-      poll.recounts
+      @recounts ||= poll.recounts
     end
 
     stats_cache(*stats_methods)
-    stats_cache :participants, :voters, :recounts
 
     def stats_cache(key, &block)
       Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/#{version}", &block)