Group replication requires GTID_MODE=ON. Group replication has an
internal check for this in group_replication_handler->start(). Thus, we
protect the call to this function with gtid_mode_lock, and we disallow
SET GTID_MODE after this function has successfully started group
replication. We detect if group replication is running by calling
is_group_replication_running.

@sql/rpl_group_replication.cc
- Take gtid_mode_lock before group_replication_handler->start.

@sql/rpl_gtid_misc.cc
- Change wrlock to rdlock since wrlock is not necessary here.

@sql/sys_vars.h
- Prevent SET GTID_MODE when group replication is running.