Remove unused lgc.interface imports and leftovers

[repository] provide a .new_session entry point The current .connect only returns a `sessionid` which must be given to repo._get_session(...) to get the real session object. Related to #4151635.

[devtools/testlib] Use actual 'admin' user configuration values Previously ``db-user`` and ``db-password`` configuration values were used to fill ``admlogin`` and `admpassword`` attributes of CubicWebTC. The correct data for these should read from `[admin]` section. Closes #4065070.

[forms] closes #2437859 - Detect and prevent concurrent edition of the same entity. Add the timestamp of form generation to each entity's meta-information fields. On form validation, check that no concurrent change is overwritten and raises a ValidationError in case of concurrent change. A nicer handling with a message and a link to the new version of the entity would be a good thing...

[migration] make sure the repo knows about all constraint types Adding new constraint types is cumbersome. It's easy to forget to do it in a migration script. So sync the constraint types at the beginning of each migration. Closes #3724892

[migration] stop caching the mapping from constraint type name to eid It's not so frequent that a cache seems necessary, and we were not invalidating that cache when adding a new constraint type. Related to #3724892.

[web/json] an empty rset is just fine for ejsonexport The ejsonexport view can just return an empty list for an empty rset. Closes #4005518