GTIDS TO SAVE AND AUTOCOMMIT=0

Problem:
=======
When autocommit is set to 0 on GR setup, it is not possible
to shutdown the R/O server. The message like below is
printed forever.

11:41.655154Z 0 [Note] InnoDB: Starting shutdown...
11:41.755413Z 0 [Note] InnoDB: Dumping buffer
pool(s) to
12:42.453581Z 0 [Note] InnoDB: Waiting for 1 active
transactions to finish

Analysis:
=========
In group replication setup when single primary mode is
enabled the secondary servers will have their
'super_read_only=ON'. During shutdown the main server thread
will try to save the executed gtid set in 'gtid_executed'
table. But since the 'super_read_only' mode is enabled on
the server the update operation fails.

When AUTOCOMMIT=0 the transaction that was started while
opening the 'gtid_executed' table doesn't complete. This
makes the main server thread to wait indefinitely for the
transaction to complete.

When AUTOCOMMIT=1, even though update to 'gtid_executed'
operation fails as 'super_read_only' is on,  it completes
the active transaction. Hence shutdwon is complete but table
is not updated.

The gtid_executed table is updated upon next server restart
as part of following code.

/*
Save unsaved GTIDs into gtid_executed table, in the following
four cases:
  1. the upgrade case.
  2. the case that a slave is provisioned from a backup of
     the master and the slave is cleaned by RESET MASTER
     and RESET SLAVE before this.
  3. the case that no binlog rotation happened from the
     last RESET MASTER on the server before it crashes.
  4. The set of GTIDs of the last binlog is not saved into
  the gtid_executed table if server crashes, so we save it
  into gtid_executed table and executed_gtids during
  recovery from the crash.
*/
if (gtid_state->save(&gtids_in_binlog_not_in_table) == -1)

Fix:
====
The main server thread should update the 'gtid_executed'
table even when 'super_read_only mode' is turned on.
'skip_readonly_check' flag is enabled now, for the thread
which tries to update executed gtid's, in the system table
during shutdown.