allow users to disable use of helper: direct uploads might be faster

[warner]

It occurred to me and Zandr that clients with fast uplinks may achieve faster throughput by not using the helper at all. The protocol improvements in #397 are nice and all, but really it's the helper that's getting overloaded. The purpose of the helper was to allow folks with slow uplinks to avoid paying the 3x expansion penalty on the wrong end of the wire, but folks with multi-megabit uplinks don't really need this help. At those speeds, sending 3x shares directly to the storage servers will get faster uploads than sending 1x ciphertext to an overloaded helper.

We might manage to farm out the helpers enough to improve performance to make this not true, but fundamentally the goal of a distributed system is to reduce centralization and push the work out to the edge. The helper is a concession to improving user experience with slow uplinks, but on fast uplinks it should probably not be used.

So the task is to give advanced end users of the commercial service a way to not use the helper. Simply deleting or renaming BASEDIR/helper.furl and restarting the node would suffice, but it might be nice to have a better UI for this. The goal is to let users with fast uplinks try it both ways and see which gives them better service.

Eventually, we might consider having the node automatically decide whether to use the helper or not, based upon the upload speeds measured by the first few direct uploads.