[ETCM-313] Integrate branch resolver actor with fast sync

Author: robinraju

src/main/scala/io/iohk/ethereum/blockchain/sync/fast/FastSync.scala

@@ -284,12 +284,26 @@ class FastSync(
       if (batchFailuresCount > fastSyncMaxBatchRetries) {
         log.info("Max skeleton batch failures reached. Master peer must be wrong.")
         handleRewind(header, masterPeer.get, fastSyncBlockValidationN, blacklistDuration)
+
+        // Start branch resolution and wait for response from the FastSyncBranchResolver actor.
+        context become waitingForBranchResolution
         branchResolver ! FastSyncBranchResolver.StartBranchResolver
-        // TODO: Become waiting for a branch resolution
         currentSkeleton = None
       }
     }
 
+    private def waitingForBranchResolution: Receive = handleCommonMessages orElse handleStatus orElse {
+      case FastSyncBranchResolver.BranchResolvedSuccessful(firstCommonBlockNumber, resolvedPeer) =>
+        // Reset the batch failures count
+        batchFailuresCount = 0
+
+        // Restart syncing from the valid block available in state.
+        syncState = syncState.copy(bestBlockHeaderNumber = firstCommonBlockNumber)
+        masterPeer = Some(resolvedPeer)
+        context become receive
+        processSyncing()
+    }
+
     private def blockHeadersError(peer: Peer) = {
       blacklist(peer.id, blacklistDuration, "error in block headers response")
       processSyncing()