helper: client should check up on the helper's work

[warner]

Really, a client which uploads a file through the Helper should turn around immediately afterwards and verify that the helper really did it's job. The most intensive approach would be to perform a full verifier run (download all shares and check all bits). A more appropriate choice might be to perform a file check plus download the UEB from all shares to make sure it matches the value returned by the helper.

Once #722 and #723 are fixed, a change like this would merely be a reliability improvement: if the helper is buggy or malicious (or not connected to the complete/correct serverlist), this would allow the client to detect it right away, while the original file is still available.

The client would respond to a helper failure by performing a direct (helperless) upload, which is obviously slower but then removes reliance upon the helper for reliability/availability.