clean up FST storage abstractions [LUCENE-4593]

I was looking at James patch for #4371, and I thought that you know, FST **almost** abstracts its underlying "i/o" (storage) via reader/writer abstractions.

It would be good to try to work on this more, e.g. we can imagine a little abstraction like lucene has a Store (Directory).

This way maybe we could cleanup the packed vs non-packed, allow for > 2GB fsts without slowing down small ones, and so on.

I have a patch that is like an amoeba-step towards this

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Migrated from [LUCENE-4593](https://issues.apache.org/jira/browse/LUCENE-4593) by Robert Muir (@rmuir), updated Jan 08 2013
Attachments: [LUCENE-4593.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4593/LUCENE-4593.patch) (versions: 2)
Sub-tasks:
- #5682