Make chunk caching less demanding on backends.

This patch modified the ChunkStore cache system so that it makes
fewer calls to the backend store. The cache uses an atom that is
updated with swap!. Previously, each swap! attempt (whether it failed or
succeeded that involved a cache miss resulted in a lookup call to the
backend, which is idempotent but a poor implementation for performance
reasons.
Merge pull request #1 from ddrichman/master

Make chunk caching less demanding on backends.