Make SingleFileStore implement NonBlockingStore

Before the introduction of NonBlockingStore, the overhead of passivation with an empty SingleFileStore was minimal, as all passivation requires is some lookups in an empty map. The introduction of NonBlockingStoreAdapter added some overhead and now operations need to switch to a blocking thread.

Implementing NonBlockingStore directly will allow keeping the map lookups on the non-blocking thread in some cases and will minimize overhead.

A NonBlockingStore should also handle segmentation on its own, and SingleFileStore could keep all the entries in one file as long as the in-memory data is split by segment.