Split out the receive_sync_changes method into a helper

This splits the method out so the bigger chunks doesn't persist the
changes in the store.

This will be useful if we need to hijack the changes and persist them in
a different store.

crates/matrix-sdk-crypto/src/machine.rs

@@ -1126,6 +1126,22 @@ impl OlmMachine {
         &self,
         sync_changes: EncryptionSyncChanges<'_>,
     ) -> OlmResult<(Vec<Raw<AnyToDeviceEvent>>, Vec<RoomKeyInfo>)> {
+        let (events, changes) = self.preprocess_sync_changes(sync_changes).await?;
+
+        // Technically save_changes also does the same work, so if it's slow we could
+        // refactor this to do it only once.
+        let room_key_updates: Vec<_> =
+            changes.inbound_group_sessions.iter().map(RoomKeyInfo::from).collect();
+
+        self.store().save_changes(changes).await?;
+
+        Ok((events, room_key_updates))
+    }
+
+    pub(crate) async fn preprocess_sync_changes(
+        &self,
+        sync_changes: EncryptionSyncChanges<'_>,
+    ) -> OlmResult<(Vec<Raw<AnyToDeviceEvent>>, Changes)> {
         // Remove verification objects that have expired or are done.
         let mut events = self.inner.verification_machine.garbage_collect();
 
@@ -1154,21 +1170,13 @@ impl OlmMachine {
             events.push(raw_event);
         }
 
-        // Technically save_changes also does the same work, so if it's slow we could
-        // refactor this to do it only once.
-        let room_key_updates: Vec<_> =
-            changes.inbound_group_sessions.iter().map(RoomKeyInfo::from).collect();
-
         let changed_sessions =
             self.inner.key_request_machine.collect_incoming_key_requests().await?;
 
         changes.sessions.extend(changed_sessions);
-
         changes.next_batch_token = sync_changes.next_batch_token;
 
-        self.store().save_changes(changes).await?;
-
-        Ok((events, room_key_updates))
+        Ok((events, changes))
     }
 
     /// Request a room key from our devices.
@@ -1419,8 +1427,8 @@ impl OlmMachine {
     ///   of tracked users
     ///
     /// Any users that hadn't been seen before will be flagged for a key query
-    /// immediately, and whenever `receive_sync_changes` receives a
-    /// "changed" notification for that user in the future.
+    /// immediately, and whenever [`OlmMachine::receive_sync_changes()`]
+    /// receives a "changed" notification for that user in the future.
     ///
     /// Users that were already in the list are unaffected.
     pub async fn update_tracked_users(