Add biz.clear() which removes all data.

This removes all cursors from the bizobj and all children. It is
intended to be used during batch-adding of new records, to keep
RAM usage constant.

Prior to this, RAM usage of my bulk-importer quickly grew to over
1.5 Gb and performance slowed to a crawl, and I had to kill the
process. After adding biz.clear(), RAM usage quickly grew to about
150 Mb and stayed around there for the duration of the bulk import,
and performance didn't degrade over time.