indicate size limits early in helper protocol

[warner]

We currently have a roughly 12GB limit on the size of an immutable file. Attempting to upload something larger than this will fail.

The helper, however, doesn't tell clients about this early: they'll push the entire ciphertext to the helper, and only then will they get an error message.

The task is: clearly define what the size limit is, expose it in the uploader code (perhaps uploader.get_max_file_size()), have the helper check it, have the helper return an error to the client if they ask to send a file larger than it.

[zooko]

see also #346 (increase share-size field to 8 bytes, remove 12GiB filesize limit) and #320 (add streaming upload to HTTP interface)

[warner]

this is biting us on the allmydata.com prodnet again. Some clients are attempting to upload 18GB+ files, which are doomed to failure, and also overflow the helper's disk.

[zooko]

I guess we don't need to do this because now there are no limits on immutable upload file size. Hooray!

[zooko]

P.S. Where by "No limits" I mean up to somewhere in the neighborhood of 2^{64 bytes.}