Fix the issue of ignoring callback calls for removed keys.

[oleksandrivantsiv]

What I did
Fix the issue of ignoring callback calls for removed keys.

Why I did it
ConfigDBConnector.listen method has a caching mechanism (added in sonic-net#587 PR) that preloads the DB state before starting. When the notification about the changed key is received the listen method gets key data from the DB (in all cases when the key was added, updated, or removed) and compares the data with the cache. It fires the callback only if data differ from the cache. Otherwise, the callback is ignored.

If the client.hgetall(key) is called for the removed key it returns an empty dictionary ({}). This can be confused with the data of the key with no attributes. For example: {"TABLE": {"KEY": {}}}.

So if preloaded data contains a key with no attributes and the listener method receives a notification about the removal of such key the callback is not called. The listener will simply remove the key from the cache without calling the callback. This leads to the situation when the removal of the key is not handled.

The solution is to get data for the added or updated keys, and for the removed keys use None instead. This will ensure that the comparison with the cache will work as expected.

How I verified it
Compile the package and run the unit test. Unit tests are extended to cover the expected behavior.

Details if related