Should not update uuid in old held leases

We've found issues when trying to update the lease with extra fields when it has been renewed by the renewer gorutine. Looking into it we've found that it is because the `concurrencyToken` is being updated (along with the whole `Lease` struct) every time it is renewed.
By comparing it with the base Java implementation I think the `concurrencyToken` should not change on `Renew`, only in `Take`.
In the commit history we've seen you already had fixed this on [commit 5737f37](https://github.com/a8m/lease/commit/5737f377b0868bd697f13f6fd0572f9ef1a515d0) yet you changed your mind on [commit e663b4a](https://github.com/a8m/lease/commit/e663b4ad70d2ddf8ec93c5ebc298a019f7a294ab); I was wondering if you could elaborate on why this last commit was needed.

If you need more info or want to comment about it please don't hesitate to contact me.

Greetings