[CWEP002] refactor rql read security checking Split 'check_read_perms' into 'check_relations_perms' which checks relations 'read' permissions and 'get_local_checks' which build dictionary of local security checks (rql expression) for variables. This allows to check relations 'read' permissions earlier in the process and so to prepare insertion of the rql rewriter: we want to check permissions of the computed relation, not permissions of relations introduced by the associated rule, to conform to the CWEP. Related to #3546717

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