Can't set user defined variable to non-trivial expressions

[systay]

mysql> set @foo = now();
ERROR 1235 (HY000): vtgate: http://andress-mbp:17825/: expression not supported for SET: now()
mysql> set @@sql_mode = now();
Query OK, 0 rows affected (0.02 sec)

We should use the same logic for user defined variables as we do for system variables, which is to delegate to the tablet to evaluate these expressions.