network: prevent disconnects during initial sync

Node receiving extensible payload from the future is confused and drops
connection. Note that this can still happen if the node is to loose its
synchrony.

Calling `IsInSync()` is quite expensive, so we stop doing that once synchrony
is reached (hence bool flag).

pkg/network/server.go

@@ -86,6 +86,7 @@ type (
 		transactions chan *transaction.Transaction
 
 		consensusStarted *atomic.Bool
+		canHandleExtens  *atomic.Bool
 
 		oracle *oracle.Oracle
 
@@ -131,6 +132,7 @@ func newServerFromConstructors(config ServerConfig, chain blockchainer.Blockchai
 		unregister:        make(chan peerDrop),
 		peers:             make(map[Peer]bool),
 		consensusStarted:  atomic.NewBool(false),
+		canHandleExtens:   atomic.NewBool(false),
 		extensiblePool:    extpool.New(chain),
 		log:               log,
 		transactions:      make(chan *transaction.Transaction, 64),
@@ -786,6 +788,12 @@ func (s *Server) handleGetHeadersCmd(p Peer, gh *payload.GetBlockByIndex) error
 
 // handleExtensibleCmd processes received extensible payload.
 func (s *Server) handleExtensibleCmd(e *payload.Extensible) error {
+	if !s.canHandleExtens.Load() {
+		if !s.IsInSync() {
+			return nil
+		}
+		s.canHandleExtens.Store(true)
+	}
 	ok, err := s.extensiblePool.Add(e)
 	if err != nil {
 		return err