[schema] fix unique together index handling We now provide a more compact indexname, using the schema constraint entity type and the position of the columns set in the entity type unique constraints list. This avoids a nasty name truncation issue. The UniqueTogetherError object is made smarter: it computes the rtypes, abstracting the underlying backend (pg/sqlserver vs sqlite). The `user friendly` adapter is much simplified since there is no longer any truncation issue. Uses a new logilab.database version (ticket #151507) and a new yams version (ticket #189299) Closes #2514939 [jcr: disable hooks when temporarily dropping CWUniqueTogetherConstraint entities]