fix header exchange init error (cosmos#962)

Fixes cosmos#846 

As per the discussion in cosmos#846, the aggregator node only need to
initialize the store with genesis header, after that it only needs to
broadcast the header without directly trying to append to store. The
append is take care by the broadcast api which along with broadcasting
also puts the header in the local append queue which then appends to the
local store.

Co-authored-by: Ganesha Upadhyaya <gupadhyaya@Ganeshas-MacBook-Pro-2.local>

node/header_exchange.go

@@ -61,18 +61,6 @@ func NewHeaderExchangeService(ctx context.Context, store ds.TxnDatastore, conf c
 	}, nil
 }
 
-func (hExService *HeaderExchangeService) initOrAppendHeaderStore(ctx context.Context, header *types.SignedHeader) error {
-	var err error
-
-	// Init the header store if first block, else append to store
-	if header.Height() == hExService.genesis.InitialHeight {
-		err = hExService.headerStore.Init(ctx, header)
-	} else {
-		err = hExService.headerStore.Append(ctx, header)
-	}
-	return err
-}
-
 func (hExService *HeaderExchangeService) initHeaderStoreAndStartSyncer(ctx context.Context, initial *types.SignedHeader) error {
 	if err := hExService.headerStore.Init(ctx, initial); err != nil {
 		return err
@@ -93,14 +81,13 @@ func (hExService *HeaderExchangeService) tryInitHeaderStoreAndStartSyncer(ctx co
 }
 
 func (hExService *HeaderExchangeService) writeToHeaderStoreAndBroadcast(ctx context.Context, signedHeader *types.SignedHeader) {
-	// Init the header store if first block, else append to store
-	if err := hExService.initOrAppendHeaderStore(ctx, signedHeader); err != nil {
-		hExService.logger.Error("failed to write block header to header store", "error", err)
-	}
-
-	// For genesis header, start the syncer
+	// For genesis header initialize the store and start the syncer
 	if signedHeader.Height() == hExService.genesis.InitialHeight {
 		if err := hExService.headerStore.Init(ctx, signedHeader); err != nil {
+			hExService.logger.Error("failed to initialize header store", "error", err)
+		}
+
+		if err := hExService.syncer.Start(hExService.ctx); err != nil {
 			hExService.logger.Error("failed to start syncer after initializing header store", "error", err)
 		}
 	}