[dataimport] add a bit of extra-documentation on the massive store about prepare_insert_relation limitation on inlined relation and lack of full-text indexation.

[dataimport] no need to call .keys()

[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).