columns data types

The "SELECT @lastId, @lastId := Id FROM t" query returns
different result sets depending on the type of the Id column
(INT or BIGINT).

Note: this fix doesn't cover the case when a select query
references an user variable and stored function that
updates a value of that variable, in this case a result
is indeterminate.


The server uses incorrect assumption about a constantness of
an user variable value as a select list item: 

The server caches a last query number where that variable
was changed and compares this number with a current query
number. If these numbers are different, the server guesses,
that the variable is not updating in the current query, so
a respective select list item is a constant. However, in some
common cases the server updates cached query number too late.


The server has been modified to memorize user variable
assignments during the parse phase to take them into account
on the next (query preparation) phase independently of the
order of user variable references/assignments in a select
item list.


mysql-test/r/user_var.result:
  Added test case for bug #26020.
mysql-test/t/user_var.test:
  Added test case for bug #26020.
sql/item_func.cc:
  An update of entry and update_query_id variables has been
  moved from Item_func_set_user_var::fix_fields() to a separate
  method, Item_func_set_user_var::set_entry().
sql/item_func.h:
  1. The Item_func_set_user_var::set_entry() method has been
  added to update Item_func_set_user_var::entry.
  
  2. The Item_func_set_user_var::entry_thd field has beend
  added to update Item_func_set_user_var::entry only when
  needed.
sql/sql_base.cc:
  Fix: setup_fiedls() calls Item_func_set_user_var::set_entry()
  for all items from the thd->lex->set_var_list before the first
  call of ::fix_fields().
sql/sql_lex.cc:
  The lex_start function has been modified to reset
  the st_lex::set_var_list list.
sql/sql_lex.h:
  New st_lex::set_var_list field has been added to
  memorize all user variable assignments in the current
  select query.
sql/sql_yacc.yy:
  The variable_aux rule has been modified to memorize
  in-query user variable assignments in the
  st_lex::set_var_list list.