Refactor the security_ctx member of the Session class

There is a publicly accessible member of the Session class call security_ctx:

  /**
    @note
    Some members of Session (currently 'Statement::db',
    'query') are set and alloced by the slave SQL thread
    (for the Session of that thread); that thread is (and must remain, for now)
    the only responsible for freeing these 3 members. If you add members
    here, and you add code to set them in replication, don't forget to
    free_them_and_set_them_to_0 in replication properly. For details see
    the 'err:' label of the handle_slave_sql() in sql/slave.cc.

    @see handle_slave_sql
  */
  Security_context security_ctx;

Tasks for refactoring project:

* Rework the Security_context class:
** Rename it to our naming standards: SecurityContext
** Make all data members private
** Give data members appropriate public accessor methods
** Remove the unused skip_grants method
** Talk with Eric Day and figure out what parts of SecurityContext overlap with the Client class
** Fix all the comments which don't apply anymore
* Refactor the Session class
** Add an accessor getSecurityContext() which returns a const reference to the SecurityContext
** Correct incorrect comments like the one shown above :)