support 'parallel' updates and rallback of whole statement in case of error in evalueting value which should be assigned
(BUG#1484)


mysql-test/r/variables.result:
  test of swaping variables
mysql-test/t/variables.test:
  test of swaping variables
sql/item_func.cc:
  new method of user variables to pre-fetch value
sql/item_func.h:
  new method of user variables to pre-fetch value
sql/set_var.cc:
  check() methods fetch & store value
sql/set_var.h:
  check() methods fetch & store value