Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

[Dawid Weiss]

> Because the reason users are working around this is Windows, where they
> need the ability to do:
>
> buffer.unmap();
> file.delete();

This is exactly the case in Lucene, for example. Which currently
resorts to ugly hacks (the entire documentation of this class is
interesting -- concerns memory fragmentation on 32-bit VMs, file
locking and other quirks).

https://github.com/apache/lucene-solr/blob/trunk/lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java#L162-L177

Dawid