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

[Dawid Weiss]

> - you track the number of mapped buffers (or mapped address space) that you
> "know" is active in the application manually.

The problem is you really can't do it on a global, JVM-scale, Peter.
It's enough that the same JVM process starts two isolated class
loaders with Lucene in each and such accounting is no longer
correct... There are also other valid reasons (the ones Uwe mentioned)
which make explicit System.gc() a non-viable option.

Dawid