[dataimport] drop massive store's flush_metadata method This method handle a temporary table which sounds useless, and even buggy in some cases (metadata for a single entity types will be only flushed once even if flush is called several time). Instead, simply call the method-doing-the job after entities have been inserted (and make it private along the way).

[dataimport] test and fix external source support for the massive store

[dataimport] add new test for nohook store importing in a custom source