[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

[dataimport] remove useless assignment in massive store

[dataimport] move eid attribution from the metagenerator back to the nohook store

[dataimport] rephrase minor bits of the meta generator