Ability to restore deleted versions

When a document is deleted a version of it is stored just before the delete happens. This version is added to the list of existing versions of the document, and then the document is deleted. In theory this should allow the document to be restored after deletion.

However currently there is no mechanism yet to restore (or find) deleted documents.

We need to add an API to retrieve the list of deleted document so that they can be restored, for example we can keep an index of these documents. The documents themselves have a `path` in the metadata which will help with the restore operation.