Also, some refactoring for app (add debug etc).

The HAL API strongly recommends to fetch paginated results using
cursors.

https://api.archives-ouvertes.fr/docs/search#cursors

A new function 'getMetadataWithCursorC' is thus added to facilitate
this.

Tests added as well

Turns out sometimes docid can be a string.

For small counts, this reported wrong number of documents (batch size
instead of the small count).

This allows to query large sets in batches and send the results
upstream.