ZF-412: Support delayed file deleting on Windows systems.
Quote from Lucene index file format description:
A file named "deletable" contains the names of files that are no longer used by the index, but which could not be deleted. This is only used on Win32, where a file may not be deleted while it is still open. On other platforms the file contains only null bytes.
Importance of this feature is increased, if we have optimization functionality finished.