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/jobs/automatic_group_membership_spec.rb

@@ -45,6 +45,7 @@ RSpec.describe Jobs::AutomaticGroupMembership do
     expect(group.users.include?(user4)).to eq(true)
     expect(group.users.include?(user5)).to eq(false)
     expect(group.users.include?(user6)).to eq(true)
+    expect(group.user_count).to eq(2)
   end
 
 end