Hash PRNG output before use, so that it's not revealed to the network

In a review of prop250, ioerror suggests that we should hash raw random values before revealing them to the network.

This avoids an attack similar to the one that broke Dual EC: http://projectbullrun.org/dual-ec/ext-rand.html

I have a patch for this, I need to clean it up to merge cleanly on top of #17686 (moved).

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