mariadb-import --use-threads can kill its own connection instead of the other pooled connections

When a worker fails in mariadb-import --use-threads=N, safe_exit() calls kill_tp_connections() to kill the other workers' connections, so the server does not keep loading after the client exits.

It sends mysql_kill() for every connection over the failing worker's own connection, in list order. The failing worker's connection is in that list, so it can kill itself first. The following mysql_kill() calls then go over a dead connection and fail (the result is not checked), and the other workers are not killed. The client then calls _exit(), and the server is left running their LOAD DATA for a client that is gone.

Seen intermittently in main.mysqldump (mariadb-import --use-threads=2 with a dropped table): the server logs "Aborted connection ... (Client disconnected)" for a worker that was still running LOAD DATA.

Fix: kill the other connections first (skip own), then mysql_close() the own connection.