download: make sure you really get all the crypttext hashes

download: make sure you really get all the crypttext hashes
We were not making sure that we really got all the crypttext hashes during download.  If a server were to return less than the complete set of crypttext hashes, then our subsequent attempt to verify the correctness of the ciphertext would fail.  (And it wouldn't be obvious without very careful debugging why it had failed.)
This patch makes it so that you keep trying to get ciphertext hashes until you have a full set or you run out of servers to ask.