VOL-1616 / VOL-1611 : Fix device state issue and memory discrepencies

- Applied mutex on node object to resolve in-memory data conflicts
- Introduced a refresh period for in-memory data

Amendments:

- Fixed node.go file format
- Bumped VERSION to 2.1.0-dev
- Use a RLock for GetLastUpdate

Change-Id: I57e88fe3ec5cceed0d4fcdb240ae9191b49ebe33
diff --git a/db/model/revision.go b/db/model/revision.go
index 74ae3f7..4e606f1 100644
--- a/db/model/revision.go
+++ b/db/model/revision.go
@@ -17,6 +17,7 @@
 
 import (
 	"github.com/opencord/voltha-go/db/kvstore"
+	"time"
 )
 
 type Revision interface {
@@ -43,6 +44,8 @@
 	GetData() interface{}
 	GetNode() *node
 	LoadFromPersistence(path string, txid string, blobs map[string]*kvstore.KVPair) []Revision
+	SetLastUpdate(ts ...time.Time)
+	GetLastUpdate() time.Time
 	UpdateData(data interface{}, branch *Branch) Revision
 	UpdateChildren(name string, children []Revision, branch *Branch) Revision
 	UpdateAllChildren(children map[string][]Revision, branch *Branch) Revision