Improve 1-letter variable names in MutableContext and implementations

Improve 1-letter variable names in MutableContext and implementations

I'm also removing obsolete short comments that don't add anything to
whats already obvious from the method header. For example, I really don't
need a comment to understand that a method that's called "setUser" and
accepts a parameter named "$user" of type "User" is meant to "set the
User object". I mean, how more obvious can it be? ;-)

Change-Id: Ib079108457a5aa2a6896e95837542ab93b465144