Sync only missing blocks on cold import

= Add eth_node_fingerprint to block that can be imitated by both hot and cold imports
- Only sync missing blocks (blocks that are missing or don't share a fingerprint) on cold import
- Set block is_final status after import

pkg/fs/reader.go

@@ -0,0 +1,14 @@
+package fs
+
+import "io/ioutil"
+
+type Reader interface {
+	Read(path string) ([]byte, error)
+}
+
+type FsReader struct {
+}
+
+func (FsReader) Read(path string) ([]byte, error) {
+	return ioutil.ReadFile(path)
+}