remove `BinaryCache.FilepathDatabase` to make cache works correct

[viruscamp]

1. remove BinaryCache.FilepathDatabase to disable loading cache by file path, fix #54
2. lazy-load cached files to reduce memory footprint (WS Shareable)
3. update and use LastAccessTime of cached files to make LRU more reasonable
4. refactoring to clear the two level cache architecture: BinaryDatabase as memory cache and BinaryCacheFolderPath as disk cache

• BinaryDatabase keeps only loaded PE, but not all file in BinaryCacheFolderPath
• remove LruCache totally, LRU is arranged by set LastAccessTime to file, Unload deletes outdated cache files by LastAccessTime