blobstore's hashing needs improvement

[Andrew Wilkins]

blobstore currently does MD5 & SHA-256, which is not any more secure than SHA-256 alone. See: http://crypto.stackexchange.com/questions/1170/best-way-to-reduce-chance-of-hash-collisions-multiple-hashes-or-larger-hash

Instead, use SHA-256 or SHA-512 alone. This must be done before we release anything using blobstore, which is about to enter as a dependency into master.

[John A Meinel (jameinel)]

This is important, but calling it Critical makes other actual Critical things less important. It would be critical if there was an actual security exploit and we needed to drop everything and fix it.

[Andrew Wilkins (axwalk)]

Sorry, wasn't meaning to suggest there was a security problem; just that we want to avoid having to migrate things unnecessarily later. This needs to be done before we release 1.21-alpha1 (assuming the code that uses blobstore lands), that's all.