Implement a filedir2 test store for dev / testing purposes

[brendanheywood]

Much of the complexity is in the objectfs mechanics and separate to which store is in use. It would be good if we implemented a filedir2 store which just shuffled files to a different directory. You'd never use it in prod but it would mean we could run a more robust suite of tests in travis with a real configured store and test it's doing the right thing. It would also lower the barrier of entry for a new dev to get started.

• [ ] Here is also a good place to add some simulated latency, both simulating slow datadir and the even slower remote store