Vdf

[nicolasochem]

Support of the new VDF daemon introduced in Kathmandu.

More context from a dev:

We have written an external daemon which monitors the chain and starts computing VDF results once the nonce revelation period of a cycle is completed. This computation is fairly intense, as in theory it is designed to be slower than the time it takes to add 32 blocks to the chain, even on the highest end machines. We have so far only tested the daemon via tezt (which, among other simplifications, has protocol parameters which are much smaller than the real protocol) so it's crucial we test it in a test network too.
Launching it should just involve launching a baker daemon with a specific command line option, e.g. ./tezos-baker-alpha -E http://127.0.0.1:16385/ run vdf . The baker simply queries the seed_computation RPC, launches a VDF computation when possible and injects the operation (which is not signed) once finished.

This PR adds support. I tried it in isolation in a private chain and it works, it's also deployed as a submod on teztnets kathmandunet right now.

We can consider adding it to mkchain private chains later, but it should only run on one baker.