Temporary variable for sub_id is 32-bit, should be 64-bit

In sql/rpl_gtid.cc, the temporary sub_id variable is declared as uint32, but should be uint64 (as sub_id is everywhere else):

 464 uint64

 465 rpl_slave_state::next_subid(uint32 domain_id)

 466 {

 467   uint32 sub_id= 0;

 468   element *elem;

 469 

 470   lock();

 471   elem= get_element(domain_id);

 472   if (elem)

 473     sub_id= ++elem->last_sub_id;

 474   unlock();

 475 

 476   return sub_id;

 477 }

That would be a fun bug to diagnose on a server with a 1-year uptime.

(Also, please rename subid to sub_id everywhere, otherwise grep is less useful.)