Add a way to force federationsender to retry sending transactions (#1077)

* Add a way to force federationsender to retry sending transactions

And use it in P2P mode when we pick up new nodes.

* Linting

* Use atomic bool to stop us blocking on the channel

federationsender/internal/perform.go

@@ -275,3 +275,16 @@ func (r *FederationSenderInternalAPI) PerformLeave(
 		request.RoomID, len(request.ServerNames),
 	)
 }
+
+// PerformServersAlive implements api.FederationSenderInternalAPI
+func (r *FederationSenderInternalAPI) PerformServersAlive(
+	ctx context.Context,
+	request *api.PerformServersAliveRequest,
+	response *api.PerformServersAliveResponse,
+) (err error) {
+	for _, srv := range request.Servers {
+		r.queues.RetryServer(srv)
+	}
+
+	return nil
+}