New metabase method IteratePendingObjects to iterate over entries in
pending_objects table. Implementation and tests are mostly copy of
code that we are using to iterate over objects table. Main difference
is that pending_objects table have StreamID column part of primary
key instead Version. Also structure of pending object is smaller
than the one from object table but it's a detail.
Method will be used to support new table in metainfo.ListObjects
request.
Next step will be to port rest of iterator implementation to support
pending_objects table in metainfo.ListPendingObjectStreams.
Part of https://github.com/storj/storj/issues/6047
Change-Id: Ia578182f88840539f3668d4a242953e061eace02
