This avoids the expensive transfer scan relying on its list of remotes
to scan being accurate throughout, which it will not be when the user
pauses syncing to a remote.
I feel it's ok to queue transfers to *any* known remote, not just the ones
being scanned.
Note that there are still small races where after syncing to a remote is
paused, a transfer can be queued for it. Not just in the expensive transfer
scan, but in the cheap failed transfer scan, and elsewhere.
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