[migration/3.21.0] performance and reliability fixes - use EXCEPT instead of NOT IN, seems to improve performance dramatically on postgresql - delete rows from cw_<etype> tables whose eid is unknown - deal with missing/dangling required inline relations (by deleting the subject entities) instead of asserting - fix/improve print statements - don't ask for confirmation before running pure select queries

[serverctl] fix source-sync command Closes #5491700

[server] missing import Fixes NameError in an unlikely error case.

[server/test] stop using email cube - make unittest_migractions use a separate datadir, and add a fake email cube there (we use it to test {add,drop}_cube, so need something that looks like a cube) - for the other test modules, just copy what we need from cubicweb-email's schema to our test schema

[web/views] Remove unused variable

[server/session] bring anonymous_connection back Fixes cubicweb-comment.

[migration/3.21] idempotency fixes Drop constraints before (re)creating them This is postgresql-specific syntax, sqlserver needs work.

[migration/3.21] Make index and table creation idempotent

[migration/3.21] Add missing foreign keys when upgrading Try to delete dangling relations, since there's no point in keeping them. Related to #4846892.

[schema2sql] properly consider skip_relations parameters. This is the same bug as yams #286912. Also, introducing a unique function telling if the relation should have a table or not unhide some other problems related to computed relation, or missing skipped relations information.