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/docker/Dockerfile.ro_core b/docker/Dockerfile.ro_core
index 8423f97..95159bd 100644
--- a/docker/Dockerfile.ro_core
+++ b/docker/Dockerfile.ro_core
@@ -36,6 +36,7 @@
 
 # Copy common files.
 COPY common ./common
+COPY db ./db
 COPY vendor ./vendor
 
 # Copy files.
diff --git a/docker/Dockerfile.rw_core b/docker/Dockerfile.rw_core
index 09a7cf6..29dabf3 100644
--- a/docker/Dockerfile.rw_core
+++ b/docker/Dockerfile.rw_core
@@ -36,6 +36,7 @@
 
 # Copy common files.
 COPY common ./common
+COPY db ./db
 COPY vendor ./vendor
 
 # Copy files