VOL-2154 : Moving back db/model to voltha-go repo

- Package imports and dockerfiles updated
- Left backend.go in voltha-lib-go

Amendments:

- Adjusted proto imports in unit tests
- Adjusted references to db/model in unit tests
- Integrate voltha-lib-go updates

Change-Id: I2d168c516a238222f0371a7bcb672d2b06796838
diff --git a/rw_core/core/device_agent.go b/rw_core/core/device_agent.go
index 5b6a7f9..3d356ec 100755
--- a/rw_core/core/device_agent.go
+++ b/rw_core/core/device_agent.go
@@ -23,8 +23,8 @@
 	"time"
 
 	"github.com/gogo/protobuf/proto"
+	"github.com/opencord/voltha-go/db/model"
 	coreutils "github.com/opencord/voltha-go/rw_core/utils"
-	"github.com/opencord/voltha-lib-go/v2/pkg/db/model"
 	fu "github.com/opencord/voltha-lib-go/v2/pkg/flows"
 	"github.com/opencord/voltha-lib-go/v2/pkg/log"
 	ic "github.com/opencord/voltha-protos/v2/go/inter_container"