More efficient if this is stored in the objectstore to avoid unnecessary
database lookups...

Allows pushing multiple protobufs per queue item, needed in the case of
listing retrieve queue jobs, which can have several VIDs in the same
job.

...before moving to first item of next job queue queue, which requires
one additional call to operator++

Summary now includes all tapepools, including those with no queued jobs
(previously we only showed a summary of queues containing jobs)