[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/logical_agent_port.go b/rw_core/core/device/logical_agent_port.go
index c82ada0..8a6e726 100644
--- a/rw_core/core/device/logical_agent_port.go
+++ b/rw_core/core/device/logical_agent_port.go
@@ -32,8 +32,8 @@
 
 // listLogicalDevicePorts returns logical device ports
 func (agent *LogicalAgent) listLogicalDevicePorts(ctx context.Context) map[uint32]*voltha.LogicalPort {
-	logger.Debug(ctx, "list-logical-device-ports")
 	portIDs := agent.portLoader.ListIDs()
+	logger.Debugw(ctx, "list-logical-device-ports", log.Fields{"num-ports": len(portIDs)})
 	ret := make(map[uint32]*voltha.LogicalPort, len(portIDs))
 	for portID := range portIDs {
 		if portHandle, have := agent.portLoader.Lock(portID); have {