Create property tests for the `BackingStore` type that are implementation-agnostic

[jorisdral]

We test BackingStore functionality at a high level through the OnDisk tests, but we do not test the full interface. Range reads remain untested, for example. As such, we should add lower-level property tests for the BackingStore type. Preferably, we test the BackingStore in an implementation-agnostic fashion, such that we can easily reuse the tests for different backing stores.