Persist blockchain with leveldb on disk (#48)

* Created test_data folder with block json files for testing + create separate file for block base.

* Fixed bug in WriteVarUint + Trim logic + unit tests

* Refactored store and add more tests for it.

* restore headerList from chain file

* Fix tx decode bug + lots of housekeeping.

* Implemented Node restore state from chain file.

* Created standalone package for storage. Added couple more methods to Batch and Store interfaces.

* Block persisting + tests

* bumped version -> 0.31.0

pkg/core/header.go

@@ -0,0 +1,38 @@
+package core
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+// Header holds the head info of a block.
+type Header struct {
+	// Base of the block.
+	BlockBase
+	// Padding that is fixed to 0
+	_ uint8
+}
+
+// DecodeBinary impelements the Payload interface.
+func (h *Header) DecodeBinary(r io.Reader) error {
+	if err := h.BlockBase.DecodeBinary(r); err != nil {
+		return err
+	}
+
+	var padding uint8
+	binary.Read(r, binary.LittleEndian, &padding)
+	if padding != 0 {
+		return fmt.Errorf("format error: padding must equal 0 got %d", padding)
+	}
+
+	return nil
+}
+
+// EncodeBinary  impelements the Payload interface.
+func (h *Header) EncodeBinary(w io.Writer) error {
+	if err := h.BlockBase.EncodeBinary(w); err != nil {
+		return err
+	}
+	return binary.Write(w, binary.LittleEndian, uint8(0))
+}