FileStore improvements

we're going to store millions of SD files in FileStore. when a directory has just a few thousands of files any access to a directory and files in it became very slow. We must think about some custom tree structure which will allow as to store manageable number of files in a directory. As all SD files are named by their hashes we can use first filename symbols to build a tree structure, like discussed here: https://stackoverflow.com/questions/1576272/storing-large-number-of-files-in-file-system