MockDirectoryWrapper should throw 'too many open files' if too many are open [LUCENE-3195]

regardless of operating system.

We should take a good default value, like the lowest one we consider 'reasonable' macosX? and it should throw exception on openInput if openFiles hashmap is too large.

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Migrated from [LUCENE-3195](https://issues.apache.org/jira/browse/LUCENE-3195) by Robert Muir (@rmuir)

Locate MockDirectoryWrapper and inspect openInput together with the openFiles hashmap. Determine a reasonable platform-independent open-file limit, then verify that opening another input past that limit reports a too-many-open-files exception regardless of operating system.

Written by the indexing model from the issue text.