unmap mapped buffer explicitly

[koron]

I borrow ByteBufferCleaner from sparkey-java.
And it is originated by Lucene

[coveralls]

Coverage decreased (-1.69%) to 90.16% when pulling e9cc6fc on koron:unmap-explicitly into fc615de on maxmind:master.

[oschwald]

Thanks! I'll have to take a closer look at the licensing and also consider whether we want to use unsupported/undocumented calls. This implementation looks reasonable though and may reduce the pain points for users where mapping locks the file.

[oschwald]

We decided against including this for the above reasons.