Implement a cache cleanup and cache-maintenance strategy

Right now, if you create the following `DataManager` stack;

```
ChunkedBloscFileDataManager → BossDBRelayDataManager
```

...then as cache-misses in the `ChunkedBloscFileDataManager` are fulfilled by the `BossDBRelayDataManager`, they're saved to disk and returned to the client.

There currently exists no mechanism by which to clear the files from `ChunkedBloscFileDataManager`, which means that the cache will grow to infinity (or until your drive is full, whichever is sooner).

There should be a cache cleanup strategy, but I believe it makes sense for there to be several strategies which a user can choose between. Perhaps options like:

* LRU
* FIFO
* Most distant (Euclidean) from LRU (??)

Even if we just implement one of these to start with, might be smart to leave space and an abstraction layer to allow for multiple in the future.