(optionally?) use redis instead of leveldb

Redis will enable horizontal scaling, which is nice.

There's the possibility of making an abstraction around redis/leveldb so that we can swap between cache implementations, but this might be overkill considering that redis is easy to install via homebrew.

No files, tests, or entry points are named. Start by locating the current LevelDB integration and the service's startup or configuration path; completion would require a decided Redis/LevelDB approach and evidence that the service can scale horizontally.

Written by the indexing model from the issue text.