add support for PERIOD_VOTE tx type

[johannbarbie]

Bounty

period vote transaction are used by validators to signal on data availability of 32-block chunks.

Scope

• enable node to handle period vote txns
• in check bridge, replace period submission with broadcast of periodSubmision txns
• in check bridge, build CAS bitmap and submit period if >2/3 of periodVotes available

Deliverables

• code
• tests

Gain for the project

• ability to run PoS

Publicly visible delivery

Roles

bounty gardener: @johannbarbie / 5%
bounty worker: @troggy / 75%
bounty reviewer: @johannbarbie / 20%

[pinkiebell]

Looks good. Interesting would be the case if we can't get 2/3 of votes for the period. How is that handled? Chain stops and waits or just move on?

[troggy]

Integration tests are out of scope and to be done here: leapdao/leap-sandbox#39

[troggy]

I've found a non-determinism in multi-validator setup: validators disagree on a few blocks per period, leading to different period roots.
This seems like a time sink, so I’d like to remove period submission from the scope of this bounty

Looks like an issue in my test setup (e.g. when one of the validator is restarted). In "normal" scenario works fine