Refactor/organize caching layer

[jfhbrook]

There are a number of concerns with the caching layer.

For one, the cache export shouldn't include all the caches. The multibundle cache isn't really related to the other caches, and multibundles should be attached to a separate abstraction than singular bundles. They should also take either a leveldb instance or a lightweight abstraction on top of leveldb, rather than taking a connstring.

Related: #135, #138, #164