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>

spec/models/group_user_spec.rb

@@ -1,6 +1,22 @@
 # frozen_string_literal: true
 
 RSpec.describe GroupUser do
+  fab!(:group) { Fabricate(:group) }
+  fab!(:user) { Fabricate(:user) }
+
+  describe 'callbacks' do
+    it "increments and decrements `Group#user_count` when record is created and destroyed" do
+      group_user = GroupUser.new(user: user, group: group)
+
+      expect do
+        group_user.save!
+      end.to change { group.reload.user_count }.from(0).to(1)
+
+      expect do
+        group_user.destroy!
+      end.to change { group.reload.user_count }.from(1).to(0)
+    end
+  end
 
   it 'correctly sets notification level' do
     moderator = Fabricate(:moderator)