The current implentation of `thread_mattr_accessor` is setting
differently-named thread variables when defining class and
instance writer methods, so the method isn't working as documented:

    Account.user = "DHH"
    Account.user # => "DHH"
    Account.new.user # => nil
    a = Account.new
    a.user = "ABC" # => "ABC"
    a.class.user # => "DHH"

At this point `:attr_Account_user` and `:attr_Class_user` thread-local
variables have been created. Modify the reader and writer methods to use
the class name instead of 'Class'.