Add light sync command

- Only syncs block headers (excludes block bodies, transactions, receipts, and logs)
- Modifies validation window to include the most recent block
- Isolates validation window to the variable defined in the cmd directory (blocks
  have a separate variable defined in the block_repository for determining when
  to set a block as final)

pkg/history/populate_headers.go

@@ -0,0 +1,29 @@
+package history
+
+import (
+	"github.com/vulcanize/vulcanizedb/pkg/core"
+	"github.com/vulcanize/vulcanizedb/pkg/datastore"
+	"log"
+)
+
+func PopulateMissingHeaders(blockchain core.Blockchain, headerRepository datastore.HeaderRepository, startingBlockNumber int64) int {
+	lastBlock := blockchain.LastBlock().Int64()
+	blockRange := headerRepository.MissingBlockNumbers(startingBlockNumber, lastBlock, blockchain.Node().ID)
+	log.SetPrefix("")
+	log.Printf("Backfilling %d blocks\n\n", len(blockRange))
+	RetrieveAndUpdateHeaders(blockchain, headerRepository, blockRange)
+	return len(blockRange)
+}
+
+func RetrieveAndUpdateHeaders(blockchain core.Blockchain, headerRepository datastore.HeaderRepository, blockNumbers []int64) int {
+	for _, blockNumber := range blockNumbers {
+		header, err := blockchain.GetHeaderByNumber(blockNumber)
+		if err != nil {
+			log.Printf("failed to retrieve block number: %d\n", blockNumber)
+			return 0
+		}
+		// TODO: handle possible error here
+		headerRepository.CreateOrUpdateHeader(header)
+	}
+	return len(blockNumbers)
+}