feat: shards mongodb driver + tests

This patch adds shard management capability to the queues mongodb
driver.

The storage API is also correctly split into an control and data
interfaces. The data drivers are loaded by default. The control
drivers are loaded as needed by the transport layer.

Unit tests are also provided to verify that the driver (and future
drivers) work as expected.

Change-Id: Iad034a429a763c9a2ce161f05c928b090ab58944
Partially-implements: blueprint storage-sharding
Partially-Closes: 1241686
Merge "feat: shards mongodb driver + tests"