PERF: Update Group#user_count counter cache outside DB transaction (#19256)

While load testing our user creation code path in production, we
identified that executing the DB statement to update the `Group#user_count` column within a
transaction is creating a bottleneck for us. This is because the
creation of a user and addition of the user to the relevant groups are
done in a transaction. When we execute the DB statement to update
`Group#user_count` for the relevant group, a row level lock is held
until the transaction completes. This row level lock acts like a global
lock when the server is creating users that will be added to the same
group in quick succession.

Instead of updating the counter cache within a transaction which the
default ActiveRecord `counter_cache` option does, we simply update the
counter cache outside of the committing transaction.

Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>

Co-authored-by: Rafael dos Santos Silva <xfalcox@gmail.com>

script/import_scripts/vbulletin.rb

@@ -195,13 +195,13 @@ class ImportScripts::VBulletin < ImportScripts::Base
         DB.exec <<~SQL
           INSERT INTO group_users (group_id, user_id, created_at, updated_at) VALUES #{values}
         SQL
-
-        Group.reset_counters(group.id, :group_users)
       rescue Exception => e
         puts e.message
         puts e.backtrace.join("\n")
       end
     end
+
+    Group.reset_all_counters!
   end
 
   def import_profile_picture(old_user, imported_user)