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/core/header.go

@@ -0,0 +1,7 @@
+package core
+
+type Header struct {
+	BlockNumber int64 `db:"block_number"`
+	Hash        string
+	Raw         []byte
+}