MDL-11640

MDL-11640
* improved handling of dirty contexts in general - caching, switching in cron, marking, loading, etc.
* role_assing() and role_unassign() now marks dirty contexts because we use has_capability() in this function - we can not do it later outside to speedup bulk operations
* fixed some inline docs
* fixed notice from rs closing
* removed cached $CONTEXT from has_capability() - $context is now required parameter; this was hiding serious bugs when context did not exist or ppl passed false as parameter
* removed some ===, we can not use these on function parameters - we must support ints, strings, '', nulls, etc. - this could be a source of really dangerous bugs in future
* some other improvements and fixes - documented inline