[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_test.go b/rw_core/core/device/logical_agent_test.go
index 94f336c..011cacb 100644
--- a/rw_core/core/device/logical_agent_test.go
+++ b/rw_core/core/device/logical_agent_test.go
@@ -264,7 +264,7 @@
 	localWG.Wait()
 	meterEntry := fu.MeterEntryFromMeterMod(ctx, meterMod)
 
-	meterHandle, have := ldAgent.meterLoader.Lock(meterMod.MeterId)
+	meterHandle, have := ldAgent.meterCache.Lock(meterMod.MeterId)
 	assert.Equal(t, have, true)
 	if have {
 		assert.True(t, proto.Equal(meterEntry, meterHandle.GetReadOnly()))