don't run a storage client if you don't need one

[zooko]

​http://tahoe-lafs.org/pipermail/tahoe-dev/2010-June/004457.html

In which Brian wrote:

Actually, it should be controlled by a tahoe.cfg knob, which defaults to True unless you've really created a server-only node. When we get a "backup service" or "repair service" implemented, in which uploads/repairs/whatever are being driven by python code inside the Tahoe node (perhaps coming from some yet-to-be-invented plugin system), we will want client functionality despite no external frontend interfaces being active.

And many "server-only" nodes will eventually want to do client-like things, for server-driven repair or rebalancing.