Fix storage and restoration of audit entries in the backup directory

The addon nuxeo-audit-storage-directory allows to persist the audit log entries in a dedicated storage (a directory).

However the Audit.Restore operation does not restore all entries, only the first one.

Steps to reproduce:

1. install the addon nuxeo-audit-storage-directory
2. start Nuxeo
3. make some changes in the application
4. observe that events are stored in the AuditStorage directory
5. drop the nuxeo-audit index in Elasticsearch
6. call the method to restore the audit

   curl -X POST 'http://localhost:8080/nuxeo/site/automation/Audit.Restore' -H 'Nuxeo-Transaction-Timeout: 3' -H 'X-NXproperties: *' -H 'X-NXRepository: default' -H 'X-NXVoidOperation: false' -H 'content-type: application/json' -d '{"params":{"auditStorage":"DirectoryAuditStorage"},"context":{}}' -u Administrator:Administrator
   

7. observe that the audit contains only 1 old entries (it may contains the new entries after Nuxeo was restarted after dropping the ES index)

Expected behavior: all old entries are restored to ES