DEV: pluck_first

Doing .pluck(:column).first is a very common pattern in Discourse and in
most cases, a limit cause isn't being added. Instead of adding a limit
clause to all these callsites, this commit adds two new methods to
ActiveRecord::Relation:

pluck_first, equivalent to limit(1).pluck(*columns).first

and pluck_first! which, like other finder methods, raises an exception
when no record is found

app/models/concerns/cached_counting.rb

@@ -58,7 +58,7 @@ module CachedCounting
     end
 
     def request_id(query_params, retries = 0)
-      id = where(query_params).pluck(:id).first
+      id = where(query_params).pluck_first(:id)
       id ||= create!(query_params.merge(count: 0)).id
     rescue # primary key violation
       if retries == 0