fix: disable fetch_existing_msgs setting by default

[link2xt]

This caused too many problems after switching the default setting for show_emails from DC_SHOW_EMAILS_OFF to DC_SHOW_EMAILS_ALL in #4019

There is a topic https://support.delta.chat/t/setting-no-chats-only-for-show-classic-e-mails-showing-classic-emails/2481 on the forum with multiple requests to revert this setting due to old emails being downloaded.

Note that this does not disable fetching existing contacts.

[link2xt]

Another option would be to revert the show_emails setting directly.

[link2xt]

There was a related fix to the bug that may have caused download of >100 messages which may have been the reason for negative reports on this feature: #4383

[link2xt]

This feature is by the way buggy because it first processes the whole Inbox, then the whole Sent folder. This way Delta Chat is unable to assign messages to the chats properly, as every time Reply-To field of the incoming message refers to the outgoing message, outgoing message is not processed yet and will only be processed after finishing the Inbox.

If we want to reintroduce fetch_existing_msgs, this should probably be a separate function that does prefetch on both the Inbox and Sent folders first, filters out messages that are >1 year old, sorts them all by internal date and downloads from oldest to newest.

[Hocuri]

Note that from a user perspective, it's not only disabled by default, but completely, since there's no way to turn it on. So, if it turns out that this isn't missed by too many users, we should just remove the code completely.

[link2xt]

Note that from a user perspective, it's not only disabled by default, but completely, since there's no way to turn it on. So, if it turns out that this isn't missed by too many users, we should just remove the code completely.

It was already effectively disabled before #4019 (enabling download of non-chat mails by default) because new users had no chat messages in the inbox and Delta Chat fetched only old chat messages. So in most cases users got no messages in the new setup and maybe only some old chat messages that happened to be unencrypted which is arguably worse than nothing. Now that this feature started doing something, users have problems when setting up a new account and get a huge number of contact requests.

The only users who could have noticed this working were DeltaLab users which already had non-chat mails enabled by default, but it was mostly used with nauta.cu which has fetch_existing_msgs=0 in the provider database.

If we reintroduce fetch_existing_msgs=1, I think we need to split it into a separate function that works on both the Inbox and Sent at the same time, makes sure not to download extremely old messages (like I got some messages dated 2020 as a contact request), downloads them sorted by internaldate, and actually test that it works nicely when setting up a new account on an existing actively used email account.