database: Only insert if there is no entry with same archive_id

archive_is is <stanza-id> or <result id=""> and should identify one
message stable and uniquely.

See XEP-0359: Unique and Stable Stanza IDs.

We need this for example for this situation:
* we go online with Profanity
* we fetch all messages since yesterday
* we add them to the db
* we go offline
* we go online with Profanity
* we fetch all messages since yesterday
* we only want to add the new ones

So far we don't ask MAM "give me all since last 'id'" but since a
certain date.

In case no archive_id will be set, it will be `(null)` and thus should
be inserted anyways because it won't find a value with (null) in that
row.

Because when adding we use `message->stanzaid ? message->stanzaid : "",`
so it will be empty in such a case.

Regards MAM: https://github.com/profanity-im/profanity/issues/660
Regards Stable IDs: https://github.com/profanity-im/profanity/issues/1207

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