optimize 4.1 codec's encoding of frequencies [LUCENE-4501]

[asfimport]

If we wanted, we could encode freq-1 into the FOR blocks (since it cannot be 0) and save some space.

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Migrated from LUCENE-4501 by Robert Muir (@rmuir)
Attachments: LUCENE-4501.patch

[asfimport]

Robert Muir (@rmuir) (migrated from JIRA)

This seems to shave about 1.5% off the .doc file in my tests.

I'm worried about making this PF confusing with these optimizations though.