[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