Refactor code into a set of modules

[josephg]

Right now all the stores are in a big monolithic directory.

They should be pulled out into small, self contained (& tested) modules. I'm not sure if it makes sense to break them into a bunch of repos or not. Maybe something like:

• Generic utilities pulled out separately. streamToIter, iterGuard
• statecraft/core: All the stores with no dependancies. Mem kv & single, router, net, map, poll?
• statecraft/*: Separate module for any of the "heavy" stores, which have more complex behaviour. Things like the foundationdb wrapper and OT stores.

This leaves a pretty heavy core. I need to make sure that specific parts can be targetted & pulled in to the browser in an isolated way that won't pull in half of the code.

The first step will be to just refactor this repo into a series of directories based on the module structure. After that I might move the code into a set of repos.