PERF: Cache serialized voters at topic view level (#28894)

This commit introduces a way to fetch the "serialized voters" for
multiple polls.

* Use a single query to fetch voters for all types of polls

* Refactor to introduce all_serialized_voters

* Cache serialized voters

plugins/poll/spec/requests/topics_controller_spec.rb

@@ -45,13 +45,9 @@ RSpec.describe PostsController do
 
         # Expected queries:
         #
-        # - all queries listed for "when not logged in"
+        # - all queries listed for "when not logged in" (except it loads voters for polls in post5, post6 and post7)
         # - query to find out if the user has voted in each poll
-        # - queries to get "serialized voters" (NOT TRACKED)
-        # - voters for poll in post5
-        # - voters for poll in post6
-        # - voters for poll in post7
-        expect(poll_queries.size).to eq(8)
+        expect(poll_queries.size).to eq(6)
       end
     end
   end