feat: accept messages from key contacts with forged From address

[link2xt]

From address is not used for key contacts
other than as the address to send replies to.

[link2xt]

This likely affects https://github.com/deltachat-bot/deltachat-loginbot as the bot authenticates email address, and now email address during securejoin can come entirely from the protected header that is not checked by the server, so sending vc-request-with-auth to the bot updated with this PR will authenticate as any email address.

[iequidoo]

What if a key contact doesn't contain a relay list in its key? receive_imf::from_field_to_contact_id() updates contacts.addr unconditionally, and for such a contact we'll only send messages to the "new" address, but it can be a replay attack aiming to break communication with the contact if its address has changed. Probably we must only update contacts.addr from newer messages, looking at the encrypted "Date". Rejecting messages with forged "From" isn't the right way to protect from such an attack, but currently it may help.