When rand() is called multiple times inside a stored procedure, the server does 
not binlog the correct random seed values.

This patch corrects the problem by resetting rand_used= 0 in 
THD::cleanup_after_query() allowing the system to save the random seeds if needed
for each command in a stored procedure body.

However, rand_used is not reset if executing in a stored function or trigger 
because these operations are binlogged by call and thus only the calling statement
need detect the call to rand() made by its substatements. These substatements must 
not set rand_used to 0 because it would remove the detection of rand() by the 
calling statement.