Verify DKG constraint system files prior to the node's start

[AnnaShaleva]

Implement DKG-required files verification prior to the node's start, as requested in #503 (review).
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[txhsl]

It's mainly about the efficiency. Reading all 6 files on startup, waiting and computing their checksums seems not the best choice.

We may need some support from https://github.com/Consensys/gnark, or add some extra information to these files.