Description
===========
This worklog introduces the following

- A dynamic privilege 'SYSTEM_USER'. The users who will have this privilege
  can be modified only through the users who will have the same privilege.
  - Security_context class is extended so that it can have privilege
    related checks.

- It will be possible to revoke the privileges from an user on specific
  databases if the user has global level grant.  This feature will be
  controlled by the system variable 'partial_revoke'. The system variable
  cannot be turned off as long as there exists one partial revokes.
  - Partial revokes are created in the user_attributes column of the mysql.user
    table.
  - Database restrictions are represented by class DB_restrictions
  - Aggregations of restrictions are done by the object from family of
    'Restrictions_aggregator' interface.
  - Overall restrictions in authorization system will be represented by the
    'Restrictions' class.
  - Please refer the UML diagrams in low level design section of the worklog
    for more details

- Not directly related :
  - A few method definitions from the Security_context class are made inline
    explicitly and some of them are moved to source files. This was done to
    de-clutter the class declarations.
  - Role_id class is renamed as Auth_id so that it can represent both Roles
    and Non-roles Auth_ids.
  - A class that manages MEM_ROOT is added in the auth_util.h
  - Security_context class now has MEM_ROOT based constructor.

Review
======
RB#21117