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/client_server/membership.rs

@@ -1033,6 +1033,11 @@ async fn join_room_by_id_helper(
 
                 drop(state_lock);
                 let pub_key_map = RwLock::new(BTreeMap::new());
+                services()
+                    .rooms
+                    .event_handler
+                    .fetch_required_signing_keys([&signed_value], &pub_key_map)
+                    .await?;
                 services()
                     .rooms
                     .event_handler
@@ -1259,6 +1264,12 @@ pub(crate) async fn invite_helper<'a>(
         )
         .map_err(|_| Error::BadRequest(ErrorKind::InvalidParam, "Origin field is invalid."))?;
 
+        services()
+            .rooms
+            .event_handler
+            .fetch_required_signing_keys([&value], &pub_key_map)
+            .await?;
+
         let pdu_id: Vec<u8> = services()
             .rooms
             .event_handler