[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.

[dataimport] add a .schema shortcut attribute on the massive store also rephrase a bit a method along the way

[dataimport] rework massive store's __init__ * _cnx is set by the parent class * count_cwuri is not used * group related things together

[dataimport] rework PGHelper class

[dataimport] flake8

[dataimport] enhance a bit sql queries readability

[dataimport] use IF EXISTS when possible it's better than try/except and avoid potential concurrency problems.

[dataimport] PGHelper should be responsible to retrieve the database schema

[dataimport] introduce a new MetadataGenerator to replace former MetaGenerator with a similar-but-different API that will allow its usage by the massive store (no more entity object manipulation), while making it a bit more flexible for customization by subclasses (experience taken from client code). Related to #10338621

[dataimport] Prefer now(pytz.utc) to utcnow

[dataimport] rename nohook store .source attribute so it's clearly not the samething as metagenerator.source. Also add a bit of doc along the way.

[dataimport] make rql store test easier to read and test a few more thing, then reuse it to test nohook start which had no tests at all