[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.

[dataimport] inline some methods of the massive store

[dataimport] move everything related to the "URI handling" feature of the massive store together

[dataimport] test for a value is in a set and insertion in a set should live together also inline some methods on the way, in the hope to make the public api clearer at some point, and only put valuable values in those sets so we don't have to endlessly test for consistency.

[dataimport] use a single _initialized set instead of _entities / _rtypes and rework a bit methods using them so that 'in' test and set insertion leave in the same method.

[dataimport] drop massive store etype to eid index and fix potential problem with is_instance_of relation insertion in case where the class of inserted entity has some (yams) parent.