[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