Adapt blob importer to work on distributed configuration

The mqueue blob importer import blobs from a MQueue into a Nuxeo blob store (blob provider), it also stores blob information (digest, key, blob provider) that are used to generate document that point to imported blob. Today the blob information storage is a directory of CSV files.

This prevent to use distributed importer using Kafka because CSV file are not shared between nodes.

This should be changed so the blob information are stored in another MQueue that can be shared in the case of Kafka MQueue.