storage/locker: add method to destroy a locker

Also renamed the collection used for the lockers.
storage: add method to check whether the instance is dying
storage: add a parameter for checking liveness in retrieve_instance
storage: improve retrieve_instance
Revert "storage: add method to check whether the instance is dying"

We won't use it.

This reverts commit c03179b4f8fdf59de27e52f01ee16e0a4f7f7fdc.