Delete a conversation

Deletes every message in a conversation, as DELETE /messages/{message_id} does for one: the conversation and its messages are removed from every list at once, attachments within 48 hours, and the phone is asked to delete the conversation's messages on its side up to the newest message deleted here (anything it holds that is newer, including texts it has not passed on yet, stays). The response returns at once, with phone_deletion.status pending while the phone has not answered; Axilio keeps checking for up to 7 days, asking the phone again until it confirms (phone_deletion.next_check_at is the next check), and a phone not yet set up to delete messages (not_default_app) is asked once it can. GET /messages/{message_id} on any of the deleted messages shows where the phone's copy stands, and each message fires message.deleted, then message.phone_deleted or message.phone_delete_failed, as DELETE /messages/{message_id} describes. Deleting a conversation does not stop a message in it the phone is still sending. A conversation with no messages left is a 404.