We were using the UploadSelectionCache previously, which does _not_ have
all nodes, or even all online nodes, in it. So all nodes with less than
MinimumVersion, or with less than MinimumDiskSpace, or nodes suspended
for unknown audit errors, or nodes that have started graceful exit, were
all missing, and ended up having empty last_nets. Even with all that,
I'm kind of surprised how many nodes this involved, but using the upload
selection cache was definitely wrong.
This change uses the download selection cache instead, which excludes
nodes only when they are disqualified, gracefully exited (completely),
or offline.
Change-Id: Iaa07c988aa29c1eb05796ac48a6f19d69f5826c1
c856d45cc0