[test/hooks] pep8 and unittest.main instead in unittest_hooks.py

[hooks/integrity] kill CheckUniqueHook It's the backend's job. The noticeable consequence (as can be seen in tests updated here) is that error message are a bit less user-friendly.

[migration] drop identity_relation if it exists It has been generated erroneously for a while, then fixed but not properly deleted from old databases. Don't use IF EXISTS for sqlserver compat.

[migration] Reserialize all constraints since yams introduce new JSON serializing format So we may drop support for older format in yams at some point.

[migration] Add IF EXISTS on DROP CONSTRAINT problem encountered on migration of a legacy database

[schema sync] Rename index when an entity type is renamed So we may still find them later using their expected name, and avoid collisions. Closes #13822045

[c-c] Add a command to check database index List indices expected by the schema but not in the database and vice-versa Closes #13822027

[sql gen] Control size of index name using an md5 hash When using concatenation of table and column, we may hit the backend size limit on index names, which results on truncated name (64 for postgres). Then we may have conflict or be unable to find expected index on later synchronization attempt. This is done by monkeypatching lgdb for now to avoid dependancy problems. Unique and primary key indices of entity types handling is left to the backend, as they are not explicitly named. See https://www.logilab.org/ticket/4789189 Closes #13821949