add interface for state syncing

syncing/interface.go

@@ -0,0 +1,13 @@
+package syncing
+
+import (
+	"github.com/harmony-one/harmony/blockchain"
+	"github.com/harmony-one/harmony/p2p"
+)
+
+// StateSync is the interface to do state-sync.
+type StateSyncInterface interface {
+	// Syncing Block in blockchain from other peers.
+	// The return channel is the signal of syncing finish.
+	ProcessStateSync(peers []p2p.Peer, bc *blockchain.Blockchain) (chan struct{}, error)
+}