DB client: documentation and behaviour

[ ] gboolean j_db_schema_get: should not return a boolean but act similar to j_kv_get etc. This returned bool gives the impression it indicates whether the schema could be retrieved/whether it existed, when in fact it indicates whether the scheduling of the operation failed.
[ ] j_db_entry_get_id: explain how the id can be used
[ ] change data type of _id (= line number in sql table) in backend from uint32 to uint64. As joins across tables are currently not favored because of missing testing etc. all variable blocks for all files are in one table. So there can be a lot of entries in the table.
[ ] is there an index on the _id field? if not please add one as this is the main identifier for reading blocks
[ ] j_db_iterator_get_field: "value points to a new allocated memory region. The caller must free this later using g_free." maybe change for consistency with other clients?
[ ] docu: how to avoid memory leaks in a while iterator loop

parcio / julea