This change add the number of documents that are soft deletes but
not hard deleted to the segment commit info. This is the last step
towards making soft deletes as powerful as hard deltes since now the
number of document can be read from commit points without opening a
full blown reader. This also allows merge posliies to make decisions
without requiring an NRT reader to get the relevant statistics. This
change doesn't enforce any field to be used as soft deletes and the statistic
is maintained per segment.