Right now the segments file stores generation, a long starting with 0
that increments by 1 with each commit, and version, a long starting
with System.currentTimeMillis() that also increments by 1 with each
commit.
I think they are redundant so we can replace all methods/uses of
version with generation instead.
Spinoff from LUCENE-1228.
Improvement
Closed
Minor
Use segments generation instead of version
