Cache chunks on disk and/or in memory

[dinowernli]

This task is about figuring out how to intelligently use disk and memory storage available in order to cache loaded chunks. The scope of this is quite large, but one way of approaching things could be:

1. Implement a very basic caching layer in the form of a CachingBackend which caches and forward calls to an actual Backend implementation. This is mostly plumbing in perparation for part (2) below.
2. Add metrics which allow evaluation of cache hit rates and expose the backend request metrics.
3. Implement caching strategies in CachingBackend which use disk and/or memory. Use the load generation binary (see other task) to validate that the strategies work.