dsbatch: New package.

This implements PutMulti and DeleteMulti with an unlimited number
of records.
reindex no longer deletes all records and recreates them.

Instead, the indexer uses the existing keys.
Use dsbatch in the main program.
During reindex, remove records if the number of packages decreased.
Sort records by package name before reindexing.

This should fix some of the churn in the ideal case.
gofmt.