Current metainfo.ListObjects implementation is using metabase iterator to list objects.
In the non-recursive case, it used to retrieve all the corresponding rows and then discarded the entries that did not fit the listing request.
This can lead in some edge cases (each prefix contains more than batchsize objects/sub-prefixes) to make unecessary calls to the db.
This change defines the metabase.ListObjects and aims at retrieving only prefixes (but not objects under it) and objects by modifying the SQL query.
In this version, it is not optimized on the database side. Cockroach will still have to go through all rows under a prefix, so there is still room for improvement.
metainfo.ListObjects is not currently using this method as we would like to assess its performance on the QA satellite first.
Fixes https://github.com/storj/storj/issues/5088
Change-Id: Ied3a9210871871d9d4a3096888d3e40c2dceed61
