Problem was that controlling connection i.e. connection that
executed the query SET GLOBAL wsrep_reject_queries = ALL_KILL;
was also killed but server would try to send result from that
query to controlling connection resulting a assertion
mysqld: /home/jan/mysql/10.2-sst/include/mysql/psi/mysql_socket.h:738: inline_mysql_socket_send: Assertion `mysql_socket.fd != -1' failed.
as socket was closed when controlling connection was closed.

wsrep_close_client_connections()
	Do not close controlling connection and instead of
	wsrep_close_thread() we do now soft kill by THD::awake

wsrep_reject_queries_update()
	Call wsrep_close_client_connections using current thd.