Refactor/organize caching layer

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

Start by tracing the cache export and the existing singular-bundle and multibundle cache abstractions, then read related issues #135, #138, and #164. Determine how the caches should be separated and how they should receive a LevelDB instance or lightweight wrapper instead of a connection string; done means the caching layer reflects those boundaries.

Written by the indexing model from the issue text.