[VOL-4178, VOL-3952] This commit removes flows/groups/meters persistency

This commit removes flows/groups/meters persistency from rw-core.
As part of this change, it also fixes a bug where devices were not
being loaded on an rw-core restart.  This is a necessary condition
to allow the non-persistency of flows/groups/meters to work.

This commit also renames "loader" to "cache" for the flows/groups/
meters to differentiate between data that is loaded from the KV
store and the one in cache.

Change-Id: Ib14e1450021abe30b17673c2910768fb740dba51
diff --git a/rw_core/core/device/transientstate/loader.go b/rw_core/core/device/transientstate/loader.go
index f8710cc..411515f 100644
--- a/rw_core/core/device/transientstate/loader.go
+++ b/rw_core/core/device/transientstate/loader.go
@@ -58,11 +58,16 @@
 	defer loader.lock.Unlock()
 
 	var deviceTransientState voltha.DeviceTransientState
-	if have, err := loader.dbProxy.Get(ctx, loader.deviceTransientState.deviceID, &deviceTransientState); err != nil || !have {
+	have, err := loader.dbProxy.Get(ctx, loader.deviceTransientState.deviceID, &deviceTransientState)
+	if err != nil {
 		logger.Errorw(ctx, "failed-to-get-device-transient-state-from-cluster-data-proxy", log.Fields{"error": err})
 		return
 	}
-	loader.deviceTransientState.transientState = deviceTransientState.TransientState
+	if have {
+		loader.deviceTransientState.transientState = deviceTransientState.TransientState
+		return
+	}
+	loader.deviceTransientState.transientState = voltha.DeviceTransientState_NONE
 }
 
 // Lock acquires the lock for deviceTransientStateLoader, and returns a handle