VOL-1775 VOL-1779 VOL-1780 : Fix several issues with overall stability

- Apply changes as reported by golang race utility
- Added version attribute in KV object
- Added context object to db/model api
- Carrying timestamp info through context to help in the
  decision making when applying a revision change
- Replaced proxy access control mechanism with etcd reservation mechanism

Change-Id: If3d142a73b1da0d64fa6a819530f297dbfada2d3
diff --git a/db/model/model.go b/db/model/model.go
index 18ff905..3446303 100644
--- a/db/model/model.go
+++ b/db/model/model.go
@@ -23,3 +23,15 @@
 	log.AddPackage(log.JSON, log.InfoLevel, log.Fields{"instanceId": "DB_MODEL"})
 	defer log.CleanUp()
 }
+
+const (
+	// period to determine when data requires a refresh (in milliseconds)
+	// TODO: make this configurable?
+	DataRefreshPeriod int64 = 5000
+
+	// Attribute used to store a timestamp in the context object
+	RequestTimestamp = "request-timestamp"
+
+	// Time limit for a KV path reservation (in seconds)
+	ReservationTTL int64 = 180
+)