Native MMapDir [LUCENE-3178]

Spinoff from #3867.

Just like we will create native Dir impl (UnixDirectory) to pass the right OS level IO flags depending on the IOContext, we could in theory do something similar with MMapDir.

The problem is MMap is apparently quite hairy... and to pass the flags the native code would need to invoke mmap (I think?), unlike UnixDir where the code "only" has to open the file handle.

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Migrated from [LUCENE-3178](https://issues.apache.org/jira/browse/LUCENE-3178) by Michael McCandless (@mikemccand), updated Mar 25 2014
Attachments: [LUCENE-3178.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3178/LUCENE-3178.patch), [LUCENE-3178-Native-MMap-implementation.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3178/LUCENE-3178-Native-MMap-implementation.patch) (versions: 3)
Linked issues:
- #3867

Start with linked issue #3867 and the attached LUCENE-3178 patches, then review how UnixDirectory, MMapDir, IOContext, and native mmap are expected to interact. Before coding, resolve the native mmap design; done means an agreed and implemented MMapDir approach that passes the appropriate OS-level I/O flags.

Written by the indexing model from the issue text.