vinyl: remove root directory if there are no more runs

When vinyl space is dropped, its files are left on the file system until GC removes them. At the moment GC removes only run files, but not the root directory. These empty directories are never removed and occupy 4KB on ext-family file systems each. In a case of many dropped vinyl spaces it can become a serious disk space and inode leak. Current commit makes gc always remove root directory if there are no runs in it.

Closes #12 (closed)