Extending cosmosStore class to support CRUD on system cosmos db

Problem:

The cosmosStore class today is the facade to perform CRUD on data partition related cosmos dbs. All the methods in this class are tightly coupled with data-partition-id. Since the system data resources won't be coupled with a data-partition-id, the existing facade can't support CRUD on system resources.

Changes in MR:

1. Enhanced the current CosmosClientFactory to return system level cosmos clients too.
2. Overloaded existing methods. The new methods don't accept data-partition-id parameter
3. Minor code refactoring

Abandoning this MR. Find new MR here: !118 (merged)