[test] Add a test for user registration unicity constraint on email

[test] Clarify user registration service tests * split permissions test appart * avoid usage of internal_cnx as this does not seem need

[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

[sql gen] Explicitly name unique index This patch hotfix logilab database for https://www.logilab.org/6662663 and fix database creation to stop using UNIQUE in the table schema but rather create the index later on, so its name is under control. Also consider this change in schema synchronization hooks. This is preliminary work related to #13822045

Update to yams 0.44 API

[test] Fix randomness in tests

[test] Drop unused import

[serverctl] allow to sync multiple and all sources in source-sync command Breaking change: by default source are not synced anymore if it is fresh (regards to synchronization-interval). To get the previous behavior an option --force is added. Closes #13886467

[tox] Split migractions tests from server environment These are the longest tests in this environment, so running them apart (in parallel of others) would hopefully speed up the whole suite.