[massive store] Reintroduce methods that are necessary to properly handle master/slave configuration Related to #15538303

[massive store] Follow configuration of the metadata generator Don't drop constraints and indexes for tables that are ignored by the metadata generator given to the store. One may now easily disable insertion of e.g. created_by / owned_by by removing them from the MetadataGenerator.META_RELATIONS set, in which case indexes for associated table won't be removed by the massive store.

[dataimport] make MetadataGenerator.META_RELATIONS customizable This should be done on the instance rather than on the class

[massive store] Rework constraint/index handling The basic idea is to keep the primary constraint on entities.eid since it's heavily used in metadata insertions. Other option would have been to drop / recreate but its very costly on big database, and the index is used for insertion into the entities table itself, so it's not worth droping it at a first glance. Also, keeping it avoids to systematically drop all constraints which depends on it. We may thus now lazily drop constraints, only on insertion of some etype/rtype for the related table. Related to #15538359

[massive store] Lazy removal of constraints and metadata indexes They should not be removed during store's init, because we may want to query the database with its index between store creation and call to prepare_insert* (e.g. to build the extid2eid map). Along the way: * rename drop_metadata_constraints into drop_metadata_indexes, because that's what it does * rework a bit impacted tests Closes #15538359

[repository] Drop the index on entities.type It should be now only used in maintenance queries in checkintegrity. Those are not worth maintaining it. Closes #15538317

[source] Drop source mapping handling It was only used by the cwxmlparser which has been deleted. This is too complex for litle benefit, we don't want to maintain that in cubicweb.

[repository] Drop the entities.extid column and associated cache This was not necessary anymore with promoted usage of the new data import API. Turn repository's _type_extid_cache to _type_cache with only the entity's type as key. This introduces an backward incompatible change: entity_metas dict doesn't contains anymore the extid key, but it doesn't seem used at all anywhere, so this sounds acceptable. Closes #15538317

[repository] Drop the entities.asource column It not used anymore thanks to previous patches. Closes #15538288

[integrity check] Stop using entities.asource column in integrity checks Simply check if entities have a cw_source, and link it to the system source when not found. Related to #15538288