Migration service on top of Nuxeo Streams

The migration service should be re-implemented using Nuxeo Streams or maybe even the bulk action framework so as to provide better resilience and scalability.

Since we have no migration to handle now, we should work on a useful sample that customers can use for their own migration. Like migrating values of a scalar property to a multi-valued one.

The expected value is simpler code in the migrator itself, for more resilience. (we may even show what would be the tag migrator code with this new implementation, as a showcase, although it might be not necessary to change the implementation now)