Trailing parts of large session command are not routed correctly

The trailing part of a large packet gets routed as if it was a normal packet. This cause the next read to hang if it gets routed to a replica. This also affects the MariaDB backend protocol module where the queued packets aren't handled correctly if the first queued packet is a COM_STMT_PREPARE.

This is because the route target is unconditionally set to TARGET_LAST_USED which causes it to not end up in handle_target_is_all().