In 4.0 livedocs are passed down to the enums, thus deleted docs are not so special.
So I think checkindex should not pass the livedocs down to the enums when checking,
it should pass livedocs=null and check all the postings. It already does this separately to
collect stats i think to compare against the term/collection statistics? But we should
just clean this up and only use one enum.
LUCENE-3876 is a case where we were actually making a corrumpt index,
(a position was negative) but because the document in question was deleted, CheckIndex
didn't detect this.
This could have caused problems if someone just passed null for livedocs (maybe they
are doing something where its not so important to take deletions into account)