Renamed simplenews_send_node() to simplenews_add_node_to_spool()

The function simplenews_send_node() did not actually send a newsletter node but only enqueued it in the mail_spool table.

Therefore, the function has been renamed to simplenews_add_node_to_spool() to make this difference clearer.

Note that sending a newsletter node directly is strongly discouraged except for test mails. To send a first add it to the mail spool and then either let it be picked up and sent during cron runs or invoke simplenews_mail_spool() to send out mails immediately. This ensures that all mails are sent exactly once, even if there are timeouts or similar issues.