feat: track gossiping per (chat, fingerprint) pair

This change simplifies
updating the gossip timestamps
when we receive a message
because we only need to know
the keys received in Autocrypt-Gossip
header and which chat the message is
assigned to.
We no longer need to iterate
over the member list.

This is a preparation
for PGP contacts
and member lists that contain
key fingerprints rather than
email addresses.

This change also removes encryption preference
from Autocrypt-Gossip header.
It SHOULD NOT be gossiped
according to the Autocrypt specification
and we ignore encryption preference anyway
since 1.157.0.

test_gossip_optimization is removed
because it relied on a per-chat gossip_timestamp.

src/peerstate.rs

@@ -405,28 +405,6 @@ impl Peerstate {
         };
     }
 
-    /// Returns the contents of the `Autocrypt-Gossip` header for outgoing messages.
-    pub fn render_gossip_header(&self, verified: bool) -> Option<String> {
-        if let Some(key) = self.peek_key(verified) {
-            let header = Aheader::new(
-                self.addr.clone(),
-                key.clone(), // TODO: avoid cloning
-                // Autocrypt 1.1.0 specification says that
-                // `prefer-encrypt` attribute SHOULD NOT be included,
-                // but we include it anyway to propagate encryption
-                // preference to new members in group chats.
-                if self.last_seen_autocrypt > 0 {
-                    self.prefer_encrypt
-                } else {
-                    EncryptPreference::NoPreference
-                },
-            );
-            Some(header.to_string())
-        } else {
-            None
-        }
-    }
-
     /// Converts the peerstate into the contact public key.
     ///
     /// Similar to [`Self::peek_key`], but consumes the peerstate and returns owned key.