The remote users need the SUPER privilege because by default Spider sends a
'SET SQL_LOG_OFF' statement to the data nodes.  This is controlled by the
spider_internal_sql_log_off configuration setting on the Spider node, which
can only be set to 0 or 1, with a default value of 1.

I have fixed the problem by changing this configuration setting so that if it
is NOT SET, which is the most likely case, the Spider node DOES NOT SEND the
'SET SQL_LOG_OFF' statement to the data nodes.  However if the
spider_internal_sql_log_off setting IS EXPLICITLY SET to either 0 or 1, then
the Spider node DOES SEND the 'SET SQL_LOG_OFF' statement, requiring a remote
user with the SUPER privilege.  The Spider documentation will be updated to
reflect this change.

Author:
  Jacob Mathew.

Reviewer:
  Kentoku Shiba.

Cherry-Picked:
  Commit 72f0efac on branch bb-10.3-MDEV-15697