Logical stores

[greglook]

Add some logical block stores which use other stores as subcomponents. Two ideas come to mind:

• [x] Caching block store which keeps up to a certain total size of block content in a cache store, backed by some authoritative primary store.
• [x] Replica store which stores blocks in multiple backing stores, ensuring at least some number succeed.

[greglook]

Added caching block store in 6ae7d04be6a366cdf8d00e513ef4d07b1b88cb8e.

[greglook]

Another idea:

• [x] A fallback lookup store which tries to look up blocks from multiple backing stores in order, returning the first result found. All writes and deletes are sent to the first store provided.

This logic also lets it be used as a way to collect a set of blocks to commit (in the vein of version control systems) by wrapping a fresh memory-store as the write layer and the main block store as the fallback. All new blobs would be collected in the memory store for review before being "committed" by writing them to the primary store.

[greglook]

Added replica storage in de1a6f63a5bcf4ab48e63e420ec992e1f7e72aa0.

[greglook]

Added buffer storage in 6ecc66d1322834925ece05949a6ad9b54edc6315.