Provide resiliency for L2 PM Data to take into account openonu-go adapter restarts

Provide resiliency for L2 PM Data to take into account openonu-go adapter restarts.

The main data that needs to be persisted on a per-onu-device and per-group-basis are the PM ME instance IDs that are active, that are waiting to be deleted and that are waiting to be deleted.

 
So the data persisted will look like this on the KV store.
Key is  }}{{<some-path-prefix>/openonu/pm-data/<onu-device-id>/<group-name>
 
The <data> thta is stored at this key will look like this

{ "instancesActive": [123456, 123457], "instanceToAdd": [123458], "instanceToDelete":[] }