assistant: When there are multiple remotes giving different ways to access the same repository, honor remote cost settings and use the cheapest available.

Note that TransferInfo does not always contain the Remote, although
any transfer added to the TransferQueue does have a Remote in its
TransferInfo. The transferkeys command still accepts a UUID, which is
useful to handle upgrades, where an old assistant version runs the new
transferkeys.

This commit was sponsored by Kalle Svensson.