Fix Common Coin use in Agreement.

[afck]

The algorithm needs a third round of messages (Conf), otherwise an adversary that controls the network scheduler and one node can potentially stall agreement: amiller/HoneyBadgerBFT#59

[vkomenda]

As suggested in the link to the description of the bug above, we use the approach of the paper "Cobalt: BFT Governance in Open Networks", https://arxiv.org/abs/1802.07240 , section 4.3.2. This is yet another important paper in the context of hbbft since it touches upon the greater problem of decentralized governance, and uses a modular protocol related to Honey Badger where the Agreement issue has been fixed.

[vkomenda]

It'd be great to come up with adversarial tests attacking the optimistic common coin schedule of #69. So that we test the difference from the paper version of Honey Badger.

[afck]

Apart from the tests, this is done. But the Conf messages might allow further optimizations.

[afck]

Closing this, since we have #228 for the missing test.