Fetch server keys concurrently

This speeds up handling of /_matrix/federation/v1/send/:transaction_id
when more than one event contains unknown keys.

In particular, when receiving multiple PDUs with dead servers in their
auth chain, timeouts of each server accumulate and can make handling of
incoming requests take several minutes, to the point the client closes
the connection (eg. matrix.org has a 2 minute timeout), causing new
events to be dropped eventually.

src/api/server_server.rs

@@ -764,6 +764,7 @@ pub async fn send_transaction_message_route(
     // events that it references.
     // let mut auth_cache = EventMap::new();
 
+    let mut parsed_pdus = vec![];
     for pdu in &body.pdus {
         let value: CanonicalJsonObject = serde_json::from_str(pdu.get()).map_err(|e| {
             warn!("Error parsing incoming event {:?}: {:?}", pdu, e);
@@ -791,8 +792,28 @@ pub async fn send_transaction_message_route(
                 continue;
             }
         };
+        parsed_pdus.push((event_id, value, room_id));
         // We do not add the event_id field to the pdu here because of signature and hashes checks
+    }
 
+    // We go through all the signatures we see on the PDUs and fetch the corresponding
+    // signing keys
+    services()
+        .rooms
+        .event_handler
+        .fetch_required_signing_keys(
+            parsed_pdus.iter().map(|(_event_id, event, _room_id)| event),
+            &pub_key_map,
+        )
+        .await
+        .unwrap_or_else(|e| {
+            warn!(
+                "Could not fetch all signatures for PDUs from {}: {:?}",
+                sender_servername, e
+            )
+        });
+
+    for (event_id, value, room_id) in parsed_pdus {
         let mutex = Arc::clone(
             services()
                 .globals