Add other checksum types

[fyrchik]

We can set it on client, sha256 is still supported.

SHA-512 seems somewhat faster on 64-bit processors:

name                 old time/op    new time/op    delta
Transformer/small-8    72.4µs ±16%    68.4µs ±12%     ~     (p=0.156 n=9+10)
Transformer/big-8       1.36s ± 8%     0.96s ± 6%  -29.27%  (p=0.000 n=10+10)

We could benchmark, and, if it makes sense, support another checksum.