Use DistributedMutex to make sure poll votes are tallied correctly.

plugins/poll/poll.rb

@@ -143,6 +143,7 @@ module ::PollPlugin
       return if is_closed?
 
       # Get the user's current vote.
+      DistributedMutex.new(details_key).synchronize do
         vote = get_vote(user)
         vote = nil unless details.keys.include? vote
 
@@ -153,6 +154,7 @@ module ::PollPlugin
         ::PluginStore.set("poll", vote_key(user), option)
         set_details! new_details
       end
+    end
 
     def serialize(user)
       return nil if details.nil?