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/sql/migrations.rs

@@ -1169,6 +1169,23 @@ CREATE INDEX msgs_status_updates_index2 ON msgs_status_updates (uid);
         .await?;
     }
 
+    inc_and_check(&mut migration_version, 130)?;
+    if dbversion < migration_version {
+        sql.execute_migration(
+            "
+CREATE TABLE gossip_timestamp (
+  chat_id INTEGER NOT NULL, 
+  fingerprint TEXT NOT NULL, -- Upper-case fingerprint of the key.
+  timestamp INTEGER NOT NULL,
+  UNIQUE (chat_id, fingerprint)
+) STRICT;
+CREATE INDEX gossip_timestamp_index ON gossip_timestamp (chat_id, fingerprint);
+",
+            migration_version,
+        )
+        .await?;
+    }
+
     let new_version = sql
         .get_raw_config_int(VERSION_CFG)
         .await?

src/sql/tables.sql

@@ -32,7 +32,7 @@ CREATE TABLE chats (
     grpid TEXT DEFAULT '',
     param TEXT DEFAULT '',
     archived INTEGER DEFAULT 0,
-    gossiped_timestamp INTEGER DEFAULT 0,
+    gossiped_timestamp INTEGER DEFAULT 0, -- deprecated 2025-04-08, replaced with gossiped_timestamp table
     locations_send_begin INTEGER DEFAULT 0,
     locations_send_until INTEGER DEFAULT 0,
     locations_last_sent INTEGER DEFAULT 0,