PoET incentivization

[mathcrypto]

There are different incentivization methods that have been discussed in the research forum. We divide them into two categories:

1. Out-of-the protocol method:
2. In protocol methods:

Right now, the out-of-protocol approach seems the best choice as it does not need protocol changes (see the discussion here)

See https://community.spacemesh.io/t/poet-incentivization/388 and https://github.com/spacemeshos/pm/issues/290

[mathcrypto]

• The current solution allows PoET servers to require a certificate (see here) obtained from a separate service ahead of time (so far provided by spacemesh).
• The certificate service will validate the PoST proof before providing an eternal certificate (doesn't expire).
• Next, this service will become more flexible, allowing some PoET operators to require payment up front, on chain, to obtain a certificate for a single epoch.
• After that stage we'll decide how to proceed and if we need an actual protocol change to support paid PoETs.
• Eventually, we're considering to add an escrow smart contract where PoETs can put up some collateral that gets distributed to smeshers who registered with that PoET in the case that their proof was never published (slashing method for bad behavior of PoET services).