MDEV-4686: Temporary variable for sub_id is 32-bit, should be 64-bit

Fix wrong type for sub_id, which would cause overflow grief on
long-running server. Also some related renames for consistency.

sql/rpl_gtid.cc

@@ -462,9 +462,9 @@ end:
 
 
 uint64
-rpl_slave_state::next_subid(uint32 domain_id)
+rpl_slave_state::next_sub_id(uint32 domain_id)
 {
-  uint32 sub_id= 0;
+  uint64 sub_id= 0;
   element *elem;
 
   lock();
@@ -722,7 +722,7 @@ rpl_slave_state::load(THD *thd, char *state_from_master, size_t len,
     uint64 sub_id;
 
     if (gtid_parser_helper(&state_from_master, end, &gtid) ||
-        !(sub_id= next_subid(gtid.domain_id)) ||
+        !(sub_id= next_sub_id(gtid.domain_id)) ||
         record_gtid(thd, &gtid, sub_id, false, in_statement) ||
         update(gtid.domain_id, gtid.server_id, sub_id, gtid.seq_no))
       return 1;

sql/rpl_gtid.h

@@ -92,7 +92,7 @@ struct rpl_slave_state
   int truncate_state_table(THD *thd);
   int record_gtid(THD *thd, const rpl_gtid *gtid, uint64 sub_id,
                   bool in_transaction, bool in_statement);
-  uint64 next_subid(uint32 domain_id);
+  uint64 next_sub_id(uint32 domain_id);
   int iterate(int (*cb)(rpl_gtid *, void *), void *data,
               rpl_gtid *extra_gtids, uint32 num_extra);
   int tostring(String *dest, rpl_gtid *extra_gtids, uint32 num_extra);

sql/slave.cc

@@ -5823,7 +5823,7 @@ static Log_event* next_event(Relay_log_info* rli)
       if (ev->get_type_code() == GTID_EVENT)
       {
         Gtid_log_event *gev= static_cast<Gtid_log_event *>(ev);
-        uint64 sub_id= rpl_global_gtid_slave_state.next_subid(gev->domain_id);
+        uint64 sub_id= rpl_global_gtid_slave_state.next_sub_id(gev->domain_id);
         if (!sub_id)
         {
           errmsg = "slave SQL thread aborted because of out-of-memory error";