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/validation_window.go

@@ -0,0 +1,41 @@
+package history
+
+import (
+	"github.com/vulcanize/vulcanizedb/pkg/core"
+	"io"
+	"text/template"
+)
+
+const WindowTemplate = `Validating Blocks
+|{{.LowerBound}}|-- Validation Window --|{{.UpperBound}}| ({{.UpperBound}}:HEAD)
+
+`
+
+var ParsedWindowTemplate = *template.Must(template.New("window").Parse(WindowTemplate))
+
+type ValidationWindow struct {
+	LowerBound int64
+	UpperBound int64
+}
+
+func (window ValidationWindow) Size() int {
+	return int(window.UpperBound - window.LowerBound)
+}
+
+func MakeValidationWindow(blockchain core.Blockchain, windowSize int) ValidationWindow {
+	upperBound := blockchain.LastBlock().Int64()
+	lowerBound := upperBound - int64(windowSize)
+	return ValidationWindow{lowerBound, upperBound}
+}
+
+func MakeRange(min, max int64) []int64 {
+	a := make([]int64, max-min+1)
+	for i := range a {
+		a[i] = min + int64(i)
+	}
+	return a
+}
+
+func (window ValidationWindow) Log(out io.Writer) {
+	ParsedWindowTemplate.Execute(out, window)
+}