feat: shards storage controller interface

Adds the storage interface for the shards storage controller. This
controller is meant to be used with the shard management queues admin
API interface.

The idea mirrors that used in writing the proxy's partition manager.

This patch also introduces the distinction between data and control
storage drivers. Data storage drivers are those that control core
functionality: messages, queues, claims. Control storage drivers are
used to manage system functionality from the point of view of an admin
- in this case, the registry of shards.

The next patch will provide an implementation for mongodb + unit tests.

Change-Id: I4f24e84c99689968e60360383b190ce168055e74
Partially-implements: blueprint storage-sharding
Partially-Closes: 1241686