WL description:
================
Most of the members of this class are public. Code accessing
and modifying them is spread in many files of MySQL code.

As part of this work log,

* All public data members of this context class are converted
  to private ones and public accessor methods are introduced
  for them. Code accessing data members directly will be replaced
  to use the getter method. And code modifying these members
  will be replaced with setter method.

* The THD class holds pointer to the object of ACL related
  context class. Accessor methods are introduced for this.
  And code accessing and modifying this member will be
  replaced with accessor methods.