RAND() must accept scalar expressions regardless of their kind.
 That includes both constant expressions and expressions that 
 depend on column values.
 When the expression is constant the random seed can be initialized
 at compile time.
 However when the expression is not constant the random seed must be
 initialized at each invocation (while it still can be allocated at
 compile time).
 Implemented the above rules by extending Item_func_rand::val_real()
 to initialize the random seed at the correct place.