need separation of private blocks and public blocks

Before v0.4.0 merges with the pinning updates, we need to be able to have blocks in our blockstore that don't get advertised and sent out to the network. Sending out pin lists would be very bad for privacy and security. This will also enable us to move towards having _all_ ipfs data stored as dags in the blockstore.

One approach to this would be to have two separate blockstore objects with different namespaces into the same underlying datastore. This way, iterating over the keys in our 'public' datastore will not yeild any private data.