make the added convergence secret be a per-file configuration

[zooko]

For some files I want convergence and I don't care about the confirmation-of-a-file attack or ​learn-partial-information attack. For others it is the other way around -- I don't care about convergence and I do care about those attacks. Therefore the value of the added convergence secret is more of a per-file configuration to me than a per-node configuration. It would be nice if tahoe put, tahoe cp, and tahoe backup offered a --converge-with= option, which would default to the node-wide added convergence secret.