private static ConcurrentMap<SegmentIdentifier, NavigableMap<Bytes, Optional<byte[]>>>
      asSegmentMap(final NavigableMap<Bytes, Optional<byte[]>> initialMap) {
    final ConcurrentMap<SegmentIdentifier, NavigableMap<Bytes, Optional<byte[]>>> segmentMap =
        new ConcurrentHashMap<>();
    segmentMap.put(SEGMENT_IDENTIFIER, initialMap);
    return segmentMap;
  }
InMemoryKeyValueStorage is just a wrapper around a single segment instance of
SegmentedInMemoryKeyValueStorage.
services/kvstore/src/main/java/org/hyperledger/besu/services/kvstore/InMemoryKeyValueStorage.java