MySQL has an SQL extension called user variables, where the user can do
something like

  SET @a := 3;
  SELECT @a + 123;

However, it also allows setting these variables within expressions:

  SELECT (@a := @a + 1) + 456;

Given that SQL does not contain any notion of sequence points, the timing of
when these variables are set is undefined. Furthermore, since they can change
_type_ at any time, semantics can become extremely confusing and/or
undefined, and it's been the source of a number of bugs.

The most popular use of such assignments is to simulate windowing functions
(e.g. row_number()), but now, we have true windowing functions.

Thus, deprecate it, staging it for later removal.

Change-Id: Ic6aa9bca2c4cb39ed8fa292401745b06fa42265c