- Have `Uniquify` take a block instead of a Proc/function. This is more
idiomatic than passing around a function in Ruby.
- Block a user before moving their issues to the ghost user. This avoids a data
race where an issue is created after the issues are migrated to the ghost user,
and before the destroy takes place.
- No need to migrate issues (to the ghost user) in a transaction, because
we're using `update_all`
- Other minor changes
1. Refactoring and specs in the `Uniquify` class.
2. Don't use the `AdvisoryLocking` class. Similar functionality is
provided (backed by Redis) in the `ExclusiveLease` class.
1. Create a `Uniquify` class, which generalizes the process of generating unique
strings, by accepting a function that defines what "uniqueness" means in a
given context.
2. WIP: Make sure tests for `Namespace` pass, add more if necessary.
3. WIP: Add tests for `Uniquify`
