[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).

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

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

[dataimport] introduce usage of MetadataGenerator into the massive store This allows more flexibility and code sharing w/ nohook store. Closes #10338621

[dataimport] rename massive store's metagen_push_relation method to _insert_meta_relation.

[dataimport] meta_data -> metadata

[dataimport] use sql shortcut

[dataimport] move cwmassive_constraint temporary table handling to the PGHelper class with consistent renaming and some rationalization to avoid doing things several times and have code slightly easier to grasp.