Previously, setting a computed relation upon entity creation or
modification (using the ORM or an RQL query) would usually fail with an
operational error in the backend ("no such table"). However, under some
mysterious circumstances (like passing a string as value in cw_set for a
computed relation), the RQL to SQL transformation would simply drop the
clause.

To prevent this to happen, we add a check for computed relation before
adding a relation to an execution plan. This check raises a QueryError.
It happens in several places:

* in querier.InsertPlan.add_relation_def() (called from several places
  in ssplanner steps) for INSERT queries,
* in ssplanner.UpdateStep.execute() for SET queries and,
* in ssplanner.SSplanner.build_delete_plan() for DELETE queries.

Tests added to unittest_querier.py because unittest_sslplanner.py looked
inappropriate (it has only unit tests) and the former already had a
NonRegressionTC class.