Add a mode to prevent duplicate documents

Such mode, if enabled through a configuration property, should force documents with a given facet to verify:

• name = filename = title (key)
• no duplicate keys in a given folder

in order to guarantee a document's "unicity" within its parent container.

Notes:

• Any attempt to create a duplicate document should throw a DuplicateNameException.
• This implies unicity of the name at the database level which is already the case for PostgreSQL since NXP-22421.
• We should be able to get rid of the deduplication mechanism adding a suffix in the PathSegmentService.
• The unicity check must be done in a privileged way. Side effect: a user may not be able to create a document because of another "invisible" document, thus an explicit message should be sent.