s3: forbid store creation when object locking is enabled and a store previously existed

[brmzkw]

Plakar refuses to create a store if the directory is not empty:

plakar at @minio create -plaintext
plakar: bucket already initialized

When hosting the store on a S3 bucket with object locking enabled, we should also check for deleted files.

Right now, the following can happen:

1. user creates a store
2. user removes all the files from the bucket
3. user created a second store
4. user accidentally removes some files
5. user wants to recover the files to a previous version: there is no way to know if these files are from the first store or the second one