netsync: change checkHeadersList behavior

2 things are changed:

1: We allow checkHeadersList to remove from the headerList even if the
   SyncManager is not in headers-first mode as we're now receiving
   headers for new block annoucements.

2: We remove headers when we no longer have checkpoints as the main
   reason we keep the last headerNode is to check that the next
   checkpoint connects.

   This is not needed on headers-first with no checkpoints as we don't
   perform that check.

   This is also not needed on new block annoucements as we will have the
   chain tip.

netsync/manager.go

@@ -811,17 +811,14 @@ func (sm *SyncManager) current() bool {
 func (sm *SyncManager) checkHeadersList(blockHash *chainhash.Hash) (
 	bool, blockchain.BehaviorFlags) {
 
-	// Nothing to check if we're not in headers-first mode.
-	if !sm.headersFirstMode {
-		return false, blockchain.BFNone
-	}
-
 	isCheckpointBlock := false
 	behaviorFlags := blockchain.BFNone
 
 	firstNodeEl := sm.headerList.Front()
 	if firstNodeEl == nil {
-		log.Warnf("headers-first mode is on but the headersList is empty")
+		if sm.headersFirstMode {
+			log.Warnf("headers-first mode is on but the headersList is empty")
+		}
 		return isCheckpointBlock, behaviorFlags
 	}
 
@@ -830,6 +827,8 @@ func (sm *SyncManager) checkHeadersList(blockHash *chainhash.Hash) (
 		return isCheckpointBlock, behaviorFlags
 	}
 
+	// When we still have checkpoints, don't remove the last header node
+	// as we need that to check if the next header connects to the list.
 	if sm.nextCheckpoint != nil {
 		behaviorFlags |= blockchain.BFFastAdd
 		if firstNode.hash.IsEqual(sm.nextCheckpoint.Hash) {
@@ -838,9 +837,11 @@ func (sm *SyncManager) checkHeadersList(blockHash *chainhash.Hash) (
 			sm.headerList.Remove(firstNodeEl)
 		}
 	} else {
-		if firstNode.height != sm.syncPeer.LastBlock() {
-			sm.headerList.Remove(firstNodeEl)
-		}
+		// Since we don't have any more checkpoints, it's ok
+		// to remove all the headers from the list as we'll
+		// reset the header state to include the tip for all
+		// the headers received when not in headers-first mode.
+		sm.headerList.Remove(firstNodeEl)
 	}
 
 	return isCheckpointBlock, behaviorFlags