If we have a partial chunk whose length is < expected chunk length, and we've just extended the length of the file, we'll need to invalidate this partial chunk by removing it from _chunkDictionary. Thus, the next time someone needs an entry from this chunk, it will be properly recreated with its correct, updated entry count.
When growing image table file, grow in increments of _entriesPerChunk.
When resetting, reset the chunk dictionary and the counted chunk set as well.