Upgrade to blocks 2.x

[greglook]

Version 2.0.0 of blocks has many changes to the storage interface. This primarily involves making the store methods asynchronous to support higher-performance and concurrent usage of the store. In addition, there have been several simplifications to the codebase.

This implementation needs to be upgraded to support the new interfaces; a simple version of this would involve wrapping futures around each of the existing methods. Listing as a stream is a bit more complex though - for pointers, see the s3 implementation.