MDL-49398 accesslib: Simplify role definition loading and cache

This change removes the complex and partial loading of role definition data for users.
It is replaced by keeping one system-wide definition for each role in MUC.

This is cheaper to regenerate as it avoids complex database queries (repeated LEFT JOIN to context).
Memory usage is kept low thanks to array CoW for each user access data.

lang/en/cache.php

@@ -63,6 +63,7 @@
 $string['cachedef_tagindexbuilder'] = 'Search results for tagged items';
 $string['cachedef_questiondata'] = 'Question definitions';
 $string['cachedef_repositories'] = 'Repositories instances data';
+$string['cachedef_roledefs'] = 'Role definitions';
 $string['cachedef_grade_categories'] = 'Grade category queries';
 $string['cachedef_string'] = 'Language string cache';
 $string['cachedef_tags'] = 'Tags collections and areas';