Almost done with the data transfer code.. Today I filled in some bits and
peices.

Made the expensive transfer scan handle multiple remotes in one pass.
So on startup, it only runs once, not N times. And when reconnecting to the
network, when a remote has changed, it scans all network remotes in one
pass, rather than making M redundant passes.

Got syncing with special remotes all working. Pretty easy actually. Just
had to avoid doing any git repo push/pull with them, while still queueing
data transfers.

It'll even download anything it can from the web special remote. To support
that, I added generic support for readonly remotes; it'll only download from
those and not try to upload to them.

(Oh, and I properly fixed the nasty `GIT_INDEX_FILE` environment variable
problem I had the other day.)

I feel I'm very close to being able to merge the assistant branch into
master now. I'm reasonably confident the data transfer code will work
well now, and manage to get things in sync eventually in all circumstances.
(Unless there are bugs.) All the other core functionality of the assistant
and webapp is working. The only think I might delay because of is the
missing [[progressbars]] in the webapp .. but that's a silly thing to
block the merge on. 

Still, I might spend a day and get a dumb implementation of progress bars
for downloads working first (progress bars for uploads are probably rather
harder). I'd spend longer on progress bars, but there are so many more
exciting things I'm now ready to develop, like automatic configurators
for using your git annex with Amazon S3, rsync.net, and the computer across
the room..!