Extend context timeout on send_join to allow for joining complex rooms (

#3153)

Background federated joins are currently broken since they timeout after
30s. This timeout didn't exist before the refactor. It should still exist but it needs to be extended to allow for the additional time it can take a server to generate the /send_join response when joining a complex room.

federationapi/internal/federationclient.go

@@ -29,7 +29,7 @@ func (a *FederationInternalAPI) MakeJoin(
 func (a *FederationInternalAPI) SendJoin(
 	ctx context.Context, origin, s spec.ServerName, event gomatrixserverlib.PDU,
 ) (res gomatrixserverlib.SendJoinResponse, err error) {
-	ctx, cancel := context.WithTimeout(ctx, defaultTimeout)
+	ctx, cancel := context.WithTimeout(ctx, time.Minute*5)
 	defer cancel()
 	ires, err := a.federation.SendJoin(ctx, origin, s, event)
 	if err != nil {