Add scroller on blob stores

We'll need to implement a new org.nuxeo.ecm.core.api.scroll.ScrollRequest passing a blob providerId, the repository name will be part of the bulk command. We may need a meta Scroller to iterate over the provider ids of a repo if many.

Be careful not allow to passing a providerId with a bad repository name (else we'll end up deleting data that should not be deleted). We can enforce this in org.nuxeo.ecm.core.bulk.BulkActionValidation ?
Do not expose a providerId param at Bulk Command level

A dryRun option could be a param of the Bulk Command.

The BulkStatus could have a new counter in addition to the total and processed counters to give information on how many blobs were effectively deleted (in this use case)

We probably want to prevent 2 concurrent scrolls for Full GCs even on the same repo and we should set sequentialCommands to true.

Let's focus on:

• S3BlobStore (see current S3BlobGarbageCollector to iterate over objects)
• LocalBlobStore (see https://docs.oracle.com/javase/7/docs/api/java/io/File.html#listFiles() to use newDirectoryStream to iterate over files