don't write corrupt >12GiB files

[warner]

I suspect that an attempt to write files that are larger than 12GiB will result in a corrupted file, as the "self._data_size" (i.e. share size) in WriteBucketProxy overflows the 4-byte space reserved for it. #346 is about removing that limit, but in the interim we need an assert or a precondition or something that will make sure we don't appear to succeed when we in fact fail.

I'm marking this as critical because it can cause data loss: you think you've uploaded the file, you get a read-cap for it, but then you can't read it back.

A precondition() in WriteBucketProxy.init would be sufficient.

[warner]

fixed, by 8c37b8e3af2f4d1b. I'm not sure what the limit is, but the new FileTooLargeError will be raised if the shares are too big for any of the fields to fit in their 4-byte containers (data_size or any of the offsets).

I think the actual size limit (i.e. the largest file you can upload) for k=3 is 12875464371 bytes. This is about 9.4MB short of 12GiB. The new assertion rejects all files which are larger than this. I don't actually know if you could upload a file of this size (is there some other limitation lurking in there?).

We still need something to prevent a client of the helper from trying to upload something this large, since it will just be a waste of time (the check that 8c37b8e3af2f4d1b adds is only for native uploads, which will be raised by the helper after they've transferred all the ciphertext over).