records_for_rebuild mysql specific batch find :all assumes integer primary key

I recommend that the following changes be made to records_for_rebuild. This selects a batch. I've tested with some large tables (>70K records) and have no problem indexing now.

offset = 1 while (rows = find :all, :limit => "#{offset},#{batch_size}").any?

offset += batch_size yield rows, offset

end

oops lost my code block

          offset = 1
          while (rows = find :all, :limit => "#{offset},#{batch_size}").any?
            offset += batch_size
            yield rows, offset
          end

I added a new option to acts_as_ferret so this feature can be turned off when using MySQL with non-integer PKs in the 'id' column:

acts_as_ferret :fields => {...}, :mysql_fast_batches => false