assist: With --jobs, parallelize transferring content to/from remotes

Command.Add.seek starts concurrency with CommandStages. And for
Command.Sync, it needs TransferStages. So, to get both types of concurrency
for the two different parts, it either needs to change the type of
concurrency in between, or just call startConcurrency once for each.

It seems safe enough to call startConcurrency twice, because it does shut
down concurrency (mostly) at the end, and eg the old Annex.workers get
emptied.

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