Delay transfer object creation until start of download
Delay transfer object creation until start of download

When fetching backups new transfer object is created for each
individual backup file. For large restoration operations there can
be 1000+ files and the transfer objects can take several gigabytes
of memory and depending on the storage type each transfer may also
require one or more file descriptors (2 for GCS), possibly causing
the app to run out of fds. The objects are not needed until the
actual download begins so only create them at that point.

ohmu/pghoard,saaros/pghoard,ohmu/pghoard