The forwardRetry RetryDecider keeps retrying a transfer as long as at least
one more byte got transferred than in the previous, failed try.

Suppose that a transfer was restarting from the beginning each time, and it
just so happened that each try got a tiny little bit further before
failing. Then transferring an `N` byte object could result in `sum [1..N]` 
bytes being sent. Worst case.

So I think forwardRetry should cap after some amount of automatic retrying.
Ie, it could give up after 5 retries. --[[Joey]]

Of course, the real use case for forwardRetry is remotes that use eg, rsync
and can really resume at the last byte. But, forwardRetry can't tell
if a remote is doing that (unless some timing heuristics were used). Around
5 retries seems fairly reasonable for that case too, it would be unlikely
for a rsync transfer to keep failing so many times while still making
forward progess. --[[Joey]]