Make update of couchdb attachments safer during import

[stropitek]

Replaced attachment

I think is possible for several attachments in an analysis to reference the same couchdb attachment.

The attachment name is determined by jpath + filename. But filename unicity within a jpath is not enforced.

Orphan attachment

I think right now we have orphan attachments when both are true:

• We replace an existing analysis attachment (same entry id + jpath + reference + attachment field)
• The filename is different

We should probably remove the previous attachment when this happens.