[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]