R1:5c84aac9089a

Allow hashers to side-grade hashes across cost settings

Summary:
Ref T4443. In addition to performing upgrades from, e.g., md5 -> bcrypt, also allow sidegrades from, e.g., bcrypt(cost=11) to bcrypt(cost=12). This allows us to, for example, bump the cost function every 18 months and stay on par with Moore's law, on average.

I'm also allowing "upgrades" which technically reduce cost, but this seems like the right thing to do (i.e., generally migrate password storage so it's all uniform, on average).

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