[schema] use json to serialize constraints
Require yams 0.43: constraints are serialized to json, which means we
need to recreate the actual checks in the database on upgrade.
Temporary deps in tox.ini to pull respective changes in yams.
option_group_changed('port', 'main', 'web')
option_group_changed('query-log-file', 'main', 'web')
option_group_changed('profile', 'main', 'web')
option_group_changed('max-post-length', 'main', 'web')